From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=113.46.200.217 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793336234; b=Tf5SYKkbMw7F6MyMi2tzI2v453/n3ClRwjlLjboJtH7rinMjMPQU7NAm0gUY/qMq9ZOr Van/Jsda7OLZCpvI0avsSR4nO3GTL3vOHKCZ6cZ1+giIxRWrUmNtSFgeXcL4M6xxR7j5o 4Dt4k4GoYFOCym3rdjyDGgNRZBCNC7P1i3bwuMM6ZZtz4m0R8GTqZsV4ZeTh1lkLe7foi stjAkiO9FTlBEQjvMd85iWtc8iZHkYf/BEbXZMsaNFfC4Y+AupaehiKJqOE1kGsCxnqgN W/SWNUafMXWVial5xgAC6brTnwMjV2aPl+aiPJ6mrW50NVyS0uBr99NmcAe5rcAcGgg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793336234; h=DMARC-Filter:dkim-signature:Received:Received:Received:Message-ID: Date:MIME-Version:User-Agent:Subject:To:CC:References:From: In-Reply-To:Content-Type:Content-Transfer-Encoding:X-Originating-IP: X-ClientProxiedBy; bh=LkSAHBtoPgrkAY7QvIyWogTu4sDPqd80TrYA0xZEnEk=; b=1sh7C6ZMPUoZbrW14qqUBNMsdjHgUN8Lv03EhuM3D8eoZtOATkky/oRK9/WRhRDzK+D5 I7BwreAnn50up8WFTHmEOH8wOFrviYWEg+EFjwX59Ze11aVZn0QsVMFsaglHTxsFQayGQ eq23zPW3Fh29HqK4IXdAAymMx7s3qniyB/Qb7aKEfgsoG3jvRDC2UMzqHVH8rRCe35fxL B3jBCy3CBbE3hsbBro9FTam1HpFGHG0LkWXv7zRO+zxcyCiRnqflVVEoA7YBhS/dfBYvB hpZp2zaiad3iZt+Iu4iI+DZMORPqZ2pSDzA5ELqG7Gsx1+iUchfRzz+W6sern7a7jyw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; dkim=pass header.d=huawei.com header.i=@huawei.com header.a=rsa-sha256 header.s=dkim header.b=dlXIONyU; arc=none smtp.remote-ip=113.46.200.217 Received: from canpmsgout02.his.huawei.com (canpmsgout02.his.huawei.com [113.46.200.217]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 12EC6606A31C; Fri, 30 Oct 2026 04:57:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 12EC6606A31C Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=huawei.com header.i=@huawei.com header.a=rsa-sha256 header.s=dkim header.b=dlXIONyU dkim-signature: v=1; a=rsa-sha256; d=huawei.com; s=dkim; c=relaxed/relaxed; q=dns/txt; h=From; bh=LkSAHBtoPgrkAY7QvIyWogTu4sDPqd80TrYA0xZEnEk=; b=dlXIONyUPuQhZmuhCVJ9Kxz1cWOkRyM210Hdd3g7CpRzTkcjoW5dcIT/e1AqdzHD3yxjPUBGt Kg8d91FT+/NcevLnn1rbWOV1WdTucGPMfWiKtOzXUsxtufmrvYHEX7AYgpa/mxbe4KxKSlvkh9H +8bUTXdstDRfB7W/SMYTv44= Received: from mail.maildlp.com (unknown [172.19.162.197]) by canpmsgout02.his.huawei.com (SkyGuard) with ESMTPS id 4jG82p6qyjzcZyh; Fri, 30 Oct 2026 12:56:50 +0800 (CST) Received: from kwepemr100010.china.huawei.com (unknown [7.202.195.125]) by mail.maildlp.com (Postfix) with ESMTPS id 24DA740579; Fri, 30 Oct 2026 12:57:08 +0800 (CST) Received: from [10.67.120.103] (10.67.120.103) by kwepemr100010.china.huawei.com (7.202.195.125) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.45; Fri, 30 Oct 2026 12:57:06 +0800 Message-ID: <5bbd0d3b-1026-43f3-bc45-245472603144@huawei.com> Date: Fri, 30 Oct 2026 12:57:06 +0800 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 08/15] KVM: arm64: Flush the HDBSS buffer on VM exit To: Leonardo Bras CC: , , , , , , , , , , , , , , , , , , , , , , , , , , , , References: <20260929103655.85107-1-zhengtian10@huawei.com> <20260929103655.85107-9-zhengtian10@huawei.com> From: Tian Zheng In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: kwepems100002.china.huawei.com (7.221.188.206) To kwepemr100010.china.huawei.com (7.202.195.125) On 10/28/2026 2:13 AM, Leonardo Bras wrote: > On Tue, Sep 29, 2026 at 06:36:48PM +0800, Tian Zheng wrote: >> From: Eillon >> >> HDBSS entries accumulate in the per-vCPU buffer while the guest runs, >> and must be drained into the dirty bitmap or dirty ring before >> userspace can observe them. >> >> Drain the buffer at a single point: kvm_arch_vcpu_ioctl_run() flushes >> it on every VM exit, before the exit reason is handled. Flushing >> inside the run loop keeps the dirty-ring feedback timely: when a flush >> pushes the ring past its soft limit, kvm_dirty_ring_push() raises >> KVM_REQ_DIRTY_RING_SOFT_FULL, which check_vcpu_requests() observes at >> the top of the next loop iteration, so the vCPU exits to userspace for >> the harvest before re-entering the guest. >> >> kvm_arch_sync_dirty_log() relies on the exit path to flush: it kicks >> vCPUs out of guest mode, and the GET/CLEAR protocol tolerates >> concurrent bitmap writes, so the snapshot is complete without extra >> synchronization. >> >> Signed-off-by: Eillon >> Signed-off-by: Tian Zheng >> --- >> arch/arm64/include/asm/kvm_dirty_bit.h | 15 +++++++++ >> arch/arm64/kvm/arm.c | 20 ++++++++++++ >> arch/arm64/kvm/dirty_bit.c | 45 ++++++++++++++++++++++++++ >> 3 files changed, 80 insertions(+) >> >> diff --git a/arch/arm64/include/asm/kvm_dirty_bit.h b/arch/arm64/include/asm/kvm_dirty_bit.h >> index fe703f02626b..d828e6b43fe9 100644 >> --- a/arch/arm64/include/asm/kvm_dirty_bit.h >> +++ b/arch/arm64/include/asm/kvm_dirty_bit.h >> @@ -13,6 +13,9 @@ >> #include >> #include >> >> +#define HDBSS_ENTRY_VALID BIT(0) >> +#define HDBSS_ENTRY_IPA GENMASK_ULL(55, 12) >> + >> #define KVM_ARM_HDBSS_DEFAULT_SIZE PAGE_SIZE >> #define KVM_ARM_HDBSS_MAX_SIZE SZ_2M >> >> @@ -22,7 +25,19 @@ static inline u32 kvm_hdbss_buffer_size(struct kvm *kvm) >> return kvm->arch.hdbss_buffer_size ?: KVM_ARM_HDBSS_DEFAULT_SIZE; >> } >> >> +static inline bool kvm_hdbss_enabled(struct kvm *kvm) >> +{ >> + return kvm->arch.mmu.vtcr & VTCR_EL2_HDBSS; >> +} >> + >> +static inline bool vcpu_hdbss_enabled(struct kvm_vcpu *vcpu) >> +{ >> + return vcpu->arch.hw_mmu && >> + (vcpu->arch.hw_mmu->vtcr & VTCR_EL2_HDBSS); >> +} >> + > > IIUC, arch.mmu is always the canonical s2_mmu, while hw_mmu can be the > nested s2_mmu, if enabled. > > Was that your intend here? > If not, we could use only kvm* version, which can be called with vcpu->kvm. > > If it was, then I would like to understand the logic here, as IIUC the > buffer is accessible only for the non-nested guest, as it's > allocated in a per-vcpu structure. (vcpu->arch) > Hi leo, Yes, that was the intent. The shadow MMU never carries VTCR_EL2.HDBSS, so the flush is naturally skipped when running nested. >> int kvm_arm_vcpu_alloc_hdbss(struct kvm_vcpu *vcpu); >> void kvm_arm_vcpu_free_hdbss(struct kvm_vcpu *vcpu); >> +void kvm_flush_hdbss_buffer(struct kvm_vcpu *vcpu); >> >> #endif /* __ARM64_KVM_DIRTY_BIT_H__ */ >> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c >> index 5ea4ac26995e..9d7bdece149a 100644 >> --- a/arch/arm64/kvm/arm.c >> +++ b/arch/arm64/kvm/arm.c >> @@ -1430,6 +1430,14 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu) >> >> trace_kvm_exit(ret, kvm_vcpu_trap_get_class(vcpu), *vcpu_pc(vcpu)); >> >> + /* >> + * Drain the HDBSS buffer before the exit is handled, so >> + * entries pushed to the dirty ring are accounted for by >> + * dirty_ring_check_request() on the next iteration. >> + */ >> + if (vcpu_hdbss_enabled(vcpu)) >> + kvm_flush_hdbss_buffer(vcpu); >> + >> /* Exit types that need handling before we can be preempted */ >> handle_exit_early(vcpu, ret); >> > > > looks the correct place, haven't looked too much on that, though. > Thanks! > >> @@ -2017,7 +2025,19 @@ long kvm_arch_vcpu_unlocked_ioctl(struct file *filp, unsigned int ioctl, >> >> void kvm_arch_sync_dirty_log(struct kvm *kvm, struct kvm_memory_slot *memslot) >> { >> + unsigned long i; >> + struct kvm_vcpu *vcpu; >> >> + if (!kvm_hdbss_enabled(kvm)) >> + return; >> + >> + /* >> + * The buffer is drained on every VM exit, so kicking running >> + * vCPUs is enough to flush them; the dirty-log GET/CLEAR >> + * protocol tolerates bits set concurrently with the snapshot. >> + */ >> + kvm_for_each_vcpu(i, vcpu, kvm) >> + kvm_vcpu_kick(vcpu); >> } >> >> static int kvm_vm_ioctl_set_device_addr(struct kvm *kvm, >> diff --git a/arch/arm64/kvm/dirty_bit.c b/arch/arm64/kvm/dirty_bit.c >> index f9aeb9f34ad0..be0d12555c84 100644 >> --- a/arch/arm64/kvm/dirty_bit.c >> +++ b/arch/arm64/kvm/dirty_bit.c >> @@ -13,6 +13,7 @@ >> #include >> #include >> #include >> +#include >> > > Out of curiosity, could you please explain you reasoning on selecting srcu > as locking here? > > (Like, what are you protecting against, and why srcu is a good choice) > kvm_vcpu_mark_page_dirty() resolves the memslot under SRCU (kvm_vcpu_gfn_to_memslot), and the flush loop calls it for each entry, so the whole loop needs the read lock held. >> int kvm_arm_vcpu_alloc_hdbss(struct kvm_vcpu *vcpu) >> { >> @@ -53,3 +54,47 @@ void kvm_arm_vcpu_free_hdbss(struct kvm_vcpu *vcpu) >> vcpu->arch.hdbss.hdbss_pg = NULL; >> vcpu->arch.hdbss.hdbssbr_el2 = 0; >> } >> + >> +void kvm_flush_hdbss_buffer(struct kvm_vcpu *vcpu) >> +{ >> + int idx, curr_idx; >> + u64 prod; >> + u32 entries; >> + u64 *hdbss_buf; >> + struct kvm *kvm = vcpu->kvm; >> + int srcu_idx; >> + >> + if (!vcpu_hdbss_enabled(vcpu)) >> + return; > > You test this before calling, is it really necessary to test it again, just > after calling? > You're right, I've removed it. Thanks, Tian >> + >> + prod = read_sysreg_s(SYS_HDBSSPROD_EL2); >> + curr_idx = HDBSSPROD_IDX(prod); >> + >> + if (curr_idx == 0 || !vcpu->arch.hdbss.hdbss_pg) >> + return; >> + >> + hdbss_buf = page_address(vcpu->arch.hdbss.hdbss_pg); >> + if (!hdbss_buf) >> + return; >> + >> + entries = kvm_hdbss_buffer_size(kvm) / sizeof(u64); >> + >> + /* kvm_vcpu_mark_page_dirty() resolves the memslot under SRCU. */ >> + srcu_idx = srcu_read_lock(&kvm->srcu); >> + for (idx = 0; idx < min_t(u32, curr_idx, entries); idx++) { >> + u64 gpa; >> + >> + gpa = hdbss_buf[idx]; >> + if (!(gpa & HDBSS_ENTRY_VALID)) >> + continue; >> + >> + gpa &= HDBSS_ENTRY_IPA; >> + kvm_vcpu_mark_page_dirty(vcpu, gpa >> PAGE_SHIFT); >> + } >> + srcu_read_unlock(&kvm->srcu, srcu_idx); >> + >> + prod &= ~HDBSSPROD_EL2_INDEX_MASK; >> + write_sysreg_s(prod, SYS_HDBSSPROD_EL2); >> + vcpu->arch.hdbss.hdbssprod_el2 = prod; >> + isb(); >> +} >> -- >> 2.43.0 >> > > > Thanks! > Leo