From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 58A613F8707; Thu, 1 Oct 2026 22:11:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=13.77.154.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790892696; cv=none; b=fLG2kM4CWpRKovEIVKAGL7KO7EkXbHmsDQxC6R21zAondl/9Tg+UnKY5oVkJOgJ+1PuJRaSApALnP/F2r1eB6xvmS36rkezTUJInUqafNcsWtIU7SMpk3CCnr7ElGvS1I34dik+N2tfN3Hpdqb3zGfW2uOhkK4pnIhZzUVbo5jw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790892696; c=relaxed/simple; bh=eqr9w/7TDzFz5BlJb4hGXHLi47k0+sqTi5KIfClTKLA=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=uvMrfJGJVZCgKNZiv15iI8JQ1Dy9tvRHLgnm13SXfHHTo/a927khrhFiSar2gEDo/sX6DlMhgtNnKM+CpHWvEm+oQo7FOPeSACLZOdiIbLV6vZpLBzyFW4ioarSMBwAy7xKlTi9XZKQzH7ILTGq/kjBvYSRf0ytOZiJZQ5d8ejY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com; spf=pass smtp.mailfrom=linux.microsoft.com; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b=Hbof5MK2; arc=none smtp.client-ip=13.77.154.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b="Hbof5MK2" Received: from linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net (linux.microsoft.com [13.77.154.182]) by linux.microsoft.com (Postfix) with ESMTPSA id BEF0120B7170; Thu, 1 Oct 2026 15:10:41 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com BEF0120B7170 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1790892641; bh=jpW55JgtvPbYS0SGoV/0R49DIok9nprOnf04hTneruc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=Hbof5MK2h9loeAOiG4ZYlJSUpIr2rZVT6CTMEluJbYHhrRcw8Xh5G1UyYTaFSzq/3 EbPasmryQcBUFnCdbFj7xHtmrAwuF79lRgFvaSMcolror1MmRReX07zFMEJIK4H0GN cM3mIyzE6xlB0l5H0fmvZwkrXd7hy4gfLMyiiERA= From: Kameron Carr To: Haiyang Zhang , Wei Liu , Dexuan Cui , Long Li , Michael Kelley Cc: linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH 3/4] Drivers: hv: vmbus: Compute ring byte counts from a caller-held snapshot Date: Thu, 1 Oct 2026 15:10:39 -0700 Message-ID: <20261001221040.1794904-4-kameroncarr@linux.microsoft.com> X-Mailer: git-send-email 2.43.7 In-Reply-To: <20261001221040.1794904-1-kameroncarr@linux.microsoft.com> References: <20261001221040.1794904-1-kameroncarr@linux.microsoft.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit hv_get_ringbuffer_availbytes() reads the indices directly, so a caller that also wants the index values has to read them a second time. The host can change them in between, resulting in the byte counts and the indices reflecting two different states of the ring. Replace it with hv_ringbuffer_avail_write() and hv_ringbuffer_avail_read(), which take the indices as arguments and return a single count. They are separate because most callers want only one of the two values. Use them in hv_ringbuffer_write() in place of the open-coded equivalent. hv_get_bytes_to_read() and hv_get_bytes_to_write() duplicated the same arithmetic, so put the helpers in include/linux/hyperv.h and use them there too. Add a hv_ringbuffer_index_valid() helper for bounds checking and use it for the checks in hv_ringbuffer_write(). Convert the remaining callers to use a single snapshot: - hv_ringbuffer_get_debuginfo() now reports the indices and the computed byte counts from the same snapshot. - hv_pkt_iter_close() computes the free space from priv_read_index instead of re-reading the shared read index. The two agree unless a misbehaving host has rewritten the value. No functional change intended for a well-behaved host. Signed-off-by: Kameron Carr --- drivers/hv/ring_buffer.c | 63 ++++++++++++++++-------------------------------- include/linux/hyperv.h | 48 +++++++++++++++++++++++++++--------- 2 files changed, 58 insertions(+), 53 deletions(-) diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c index 7f466c5..29edab9 100644 --- a/drivers/hv/ring_buffer.c +++ b/drivers/hv/ring_buffer.c @@ -107,35 +107,12 @@ static u32 hv_copyto_ringbuffer( return start_write_offset; } -/* - * - * hv_get_ringbuffer_availbytes() - * - * Get number of bytes available to read and to write to - * for the specified ring buffer - */ -static void -hv_get_ringbuffer_availbytes(const struct hv_ring_buffer_info *rbi, - u32 *read, u32 *write) -{ - u32 read_loc, write_loc, dsize; - - /* Capture the read/write indices before they changed */ - read_loc = READ_ONCE(rbi->ring_buffer->read_index); - write_loc = READ_ONCE(rbi->ring_buffer->write_index); - dsize = rbi->ring_datasize; - - *write = write_loc >= read_loc ? dsize - (write_loc - read_loc) : - read_loc - write_loc; - *read = dsize - *write; -} - /* Get various debug metrics for the specified ring buffer. */ int hv_ringbuffer_get_debuginfo(struct hv_ring_buffer_info *ring_info, struct hv_ring_buffer_debug_info *debug_info) { - u32 bytes_avail_towrite; - u32 bytes_avail_toread; + u32 read_index; + u32 write_index; mutex_lock(&ring_info->ring_buffer_mutex); @@ -144,13 +121,14 @@ int hv_ringbuffer_get_debuginfo(struct hv_ring_buffer_info *ring_info, return -EINVAL; } - hv_get_ringbuffer_availbytes(ring_info, - &bytes_avail_toread, - &bytes_avail_towrite); - debug_info->bytes_avail_toread = bytes_avail_toread; - debug_info->bytes_avail_towrite = bytes_avail_towrite; - debug_info->current_read_index = ring_info->ring_buffer->read_index; - debug_info->current_write_index = ring_info->ring_buffer->write_index; + read_index = READ_ONCE(ring_info->ring_buffer->read_index); + write_index = READ_ONCE(ring_info->ring_buffer->write_index); + debug_info->bytes_avail_toread = + hv_ringbuffer_avail_read(ring_info, read_index, write_index); + debug_info->bytes_avail_towrite = + hv_ringbuffer_avail_write(ring_info, read_index, write_index); + debug_info->current_read_index = read_index; + debug_info->current_write_index = write_index; debug_info->current_interrupt_mask = ring_info->ring_buffer->interrupt_mask; mutex_unlock(&ring_info->ring_buffer_mutex); @@ -282,8 +260,8 @@ int hv_ringbuffer_write(struct vmbus_channel *channel, read_index = READ_ONCE(outring_info->ring_buffer->read_index); old_write = READ_ONCE(outring_info->ring_buffer->write_index); - if (unlikely(read_index >= outring_info->ring_datasize || - old_write >= outring_info->ring_datasize)) { + if (unlikely(!hv_ringbuffer_index_valid(outring_info, read_index) || + !hv_ringbuffer_index_valid(outring_info, old_write))) { spin_unlock_irqrestore(&outring_info->ring_lock, flags); pr_err_ratelimited("outbound ring indices out of range: relid %u read %u write %u size %u\n", channel->offermsg.child_relid, read_index, @@ -291,9 +269,8 @@ int hv_ringbuffer_write(struct vmbus_channel *channel, return -EIO; } - bytes_avail_towrite = old_write >= read_index ? - outring_info->ring_datasize - (old_write - read_index) : - read_index - old_write; + bytes_avail_towrite = hv_ringbuffer_avail_write(outring_info, read_index, + old_write); /* * If there is only room for the packet, assume it is full. @@ -568,6 +545,7 @@ void hv_pkt_iter_close(struct vmbus_channel *channel) { struct hv_ring_buffer_info *rbi = &channel->inbound; u32 curr_write_sz, pending_sz, bytes_read, start_read_index; + u32 write_index; /* * Make sure all reads are done before we update the read index since @@ -607,11 +585,13 @@ void hv_pkt_iter_close(struct vmbus_channel *channel) return; /* - * Ensure the read of write_index in hv_get_bytes_to_write() - * happens after the read of pending_send_sz. + * Ensure the read of write_index happens after the read of + * pending_send_sz. */ virt_rmb(); - curr_write_sz = hv_get_bytes_to_write(rbi); + write_index = READ_ONCE(rbi->ring_buffer->write_index); + curr_write_sz = hv_ringbuffer_avail_write(rbi, rbi->priv_read_index, + write_index); bytes_read = hv_pkt_iter_bytes_read(rbi, start_read_index); /* @@ -627,8 +607,7 @@ void hv_pkt_iter_close(struct vmbus_channel *channel) * Exactly filling the ring buffer is treated as "not enough * space". The ring buffer always must have at least one byte * empty so the empty and full conditions are distinguishable. - * hv_get_bytes_to_write() doesn't fully tell the truth in - * this regard. + * curr_write_sz doesn't fully tell the truth in this regard. * * So first check if we were in the "enough free space" state * before we began the iteration. If so, the host was not diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h index 5c65820..9d7d09c 100644 --- a/include/linux/hyperv.h +++ b/include/linux/hyperv.h @@ -209,31 +209,57 @@ struct hv_ring_buffer_info { }; +/* + * The indices live in memory shared with the untrusted host, so check one + * before using it as an offset or to compute a byte count. + */ +static inline bool +hv_ringbuffer_index_valid(const struct hv_ring_buffer_info *rbi, u32 index) +{ + return index < rbi->ring_datasize; +} + +/* + * Byte counts for a caller-supplied snapshot of the indices, so that the + * counts and the indices the caller goes on to use describe one state of the + * ring. + */ +static inline u32 +hv_ringbuffer_avail_write(const struct hv_ring_buffer_info *rbi, + u32 read_loc, u32 write_loc) +{ + u32 dsize = rbi->ring_datasize; + + return write_loc >= read_loc ? dsize - (write_loc - read_loc) : + read_loc - write_loc; +} + +static inline u32 +hv_ringbuffer_avail_read(const struct hv_ring_buffer_info *rbi, + u32 read_loc, u32 write_loc) +{ + return rbi->ring_datasize - + hv_ringbuffer_avail_write(rbi, read_loc, write_loc); +} + static inline u32 hv_get_bytes_to_read(const struct hv_ring_buffer_info *rbi) { - u32 read_loc, write_loc, dsize, read; + u32 read_loc, write_loc; - dsize = rbi->ring_datasize; read_loc = READ_ONCE(rbi->ring_buffer->read_index); write_loc = READ_ONCE(rbi->ring_buffer->write_index); - read = write_loc >= read_loc ? (write_loc - read_loc) : - (dsize - read_loc) + write_loc; - - return read; + return hv_ringbuffer_avail_read(rbi, read_loc, write_loc); } static inline u32 hv_get_bytes_to_write(const struct hv_ring_buffer_info *rbi) { - u32 read_loc, write_loc, dsize, write; + u32 read_loc, write_loc; - dsize = rbi->ring_datasize; read_loc = READ_ONCE(rbi->ring_buffer->read_index); write_loc = READ_ONCE(rbi->ring_buffer->write_index); - write = write_loc >= read_loc ? dsize - (write_loc - read_loc) : - read_loc - write_loc; - return write; + return hv_ringbuffer_avail_write(rbi, read_loc, write_loc); } static inline u32 hv_get_avail_to_write_percent( -- 2.45.4