From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0860DC433EF for ; Sun, 17 Oct 2021 07:54:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D92E560F11 for ; Sun, 17 Oct 2021 07:54:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241495AbhJQH4n (ORCPT ); Sun, 17 Oct 2021 03:56:43 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:36538 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233999AbhJQH4h (ORCPT ); Sun, 17 Oct 2021 03:56:37 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1634457267; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=+0osY3/SAm8vFbOEyznQ3BBtqYdhl6eYu7QQs/YzIGE=; b=bhtjJvOCBwpW4PmCSKdCGOupZZ9Nb/GVVIshEkwd7zm2ToQK2dWEoUEXOcdgSgKq8Y82GU wtRbUPbG9VkRLQJYWidNif3U2REMf+7rKk3ISPLbu5uj0mYY+zMxCWBLJ9dYJOjcYmek0Q H+2umQ2qGL2hThiIiP7eCYqJSNfhbaY= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-354-lYQW2WO-P8Cx1coOwejEIA-1; Sun, 17 Oct 2021 03:54:22 -0400 X-MC-Unique: lYQW2WO-P8Cx1coOwejEIA-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 03F3B800685; Sun, 17 Oct 2021 07:54:21 +0000 (UTC) Received: from starship (unknown [10.40.192.246]) by smtp.corp.redhat.com (Postfix) with ESMTP id 2F78F5C1C5; Sun, 17 Oct 2021 07:54:17 +0000 (UTC) Message-ID: Subject: Re: [PATCH RFC] KVM: SVM: reduce guest MAXPHYADDR by one in case C-bit is a physical bit From: Maxim Levitsky To: Sean Christopherson , Vitaly Kuznetsov Cc: kvm@vger.kernel.org, Paolo Bonzini , Wanpeng Li , Jim Mattson , Tom Lendacky , David Matlack , linux-kernel@vger.kernel.org Date: Sun, 17 Oct 2021 10:54:16 +0300 In-Reply-To: References: <20211015150524.2030966-1-vkuznets@redhat.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.5 (3.36.5-2.fc32) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2021-10-15 at 15:24 +0000, Sean Christopherson wrote: > On Fri, Oct 15, 2021, Vitaly Kuznetsov wrote: > > Several selftests (memslot_modification_stress_test, kvm_page_table_test, > > dirty_log_perf_test,.. ) which rely on vm_get_max_gfn() started to fail > > since commit ef4c9f4f65462 ("KVM: selftests: Fix 32-bit truncation of > > vm_get_max_gfn()") on AMD EPYC 7401P: > > > > ./tools/testing/selftests/kvm/demand_paging_test > > Testing guest mode: PA-bits:ANY, VA-bits:48, 4K pages > > guest physical test memory offset: 0xffffbffff000 > > This look a lot like the signature I remember from the original bug[1]. I assume > you're hitting the magic HyperTransport region[2]. I thought that was fixed, but > the hack-a-fix for selftests never got applied[3]. Hi Vitaly and everyone! You are the 3rd person to suffer from this issue :-( Sean Christopherson was first, I was second. I reported this, then I think we found out that it is not the HyperTransport region after all, and I think that the whole thing lost in 'trying to get answers from AMD'. https://lore.kernel.org/lkml/ac72b77c-f633-923b-8019-69347db706be@redhat.com/ I'll say, a hack to reduce it by 1 bit is still better that failing tests, at least until AMD explains to us, about what is going on. Sorry that you had to debug this. Best regards, Maxim Levitsky > > [1] https://lore.kernel.org/lkml/20210623230552.4027702-4-seanjc@google.com/ > [2] https://lkml.kernel.org/r/7e3a90c0-75a1-b8fe-dbcf-bda16502ace9@amd.com > [3] https://lkml.kernel.org/r/20210805105423.412878-1-pbonzini@redhat.com > > > Finished creating vCPUs and starting uffd threads > > Started all vCPUs > > ==== Test Assertion Failure ==== > > demand_paging_test.c:63: false > > pid=47131 tid=47134 errno=0 - Success > > 1 0x000000000040281b: vcpu_worker at demand_paging_test.c:63 > > 2 0x00007fb36716e431: ?? ??:0 > > 3 0x00007fb36709c912: ?? ??:0 > > Invalid guest sync status: exit_reason=SHUTDOWN > > > > The commit, however, seems to be correct, it just revealed an already > > present issue. AMD CPUs which support SEV may have a reduced physical > > address space, e.g. on AMD EPYC 7401P I see: > > > > Address sizes: 43 bits physical, 48 bits virtual > > > > The guest physical address space, however, is not reduced as stated in > > commit e39f00f60ebd ("KVM: x86: Use kernel's x86_phys_bits to handle > > reduced MAXPHYADDR"). This seems to be almost correct, however, APM has one > > more clause (15.34.6): > > > > Note that because guest physical addresses are always translated through > > the nested page tables, the size of the guest physical address space is > > not impacted by any physical address space reduction indicated in CPUID > > 8000_001F[EBX]. If the C-bit is a physical address bit however, the guest > > physical address space is effectively reduced by 1 bit. > > > > Implement the reduction. > > > > Fixes: e39f00f60ebd (KVM: x86: Use kernel's x86_phys_bits to handle reduced MAXPHYADDR) > > Signed-off-by: Vitaly Kuznetsov > > --- > > - RFC: I may have misdiagnosed the problem as I didn't dig to where exactly > > the guest crashes. > > --- > > arch/x86/kvm/cpuid.c | 13 ++++++++++--- > > 1 file changed, 10 insertions(+), 3 deletions(-) > > > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > > index 751aa85a3001..04ae280a0b66 100644 > > --- a/arch/x86/kvm/cpuid.c > > +++ b/arch/x86/kvm/cpuid.c > > @@ -923,13 +923,20 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function) > > * > > * If TDP is enabled but an explicit guest MAXPHYADDR is not > > * provided, use the raw bare metal MAXPHYADDR as reductions to > > - * the HPAs do not affect GPAs. > > + * the HPAs do not affect GPAs. The value, however, has to be > > + * reduced by 1 in case C-bit is a physical bit (APM section > > + * 15.34.6). > > */ > > - if (!tdp_enabled) > > + if (!tdp_enabled) { > > g_phys_as = boot_cpu_data.x86_phys_bits; > > - else if (!g_phys_as) > > + } else if (!g_phys_as) { > > g_phys_as = phys_as; > > > > + if (kvm_cpu_cap_has(X86_FEATURE_SEV) && > > + (cpuid_ebx(0x8000001f) & 0x3f) < g_phys_as) > > + g_phys_as -= 1; > > This is incorrect, non-SEV guests do not see a reduced address space. See Tom's > explanation[*] > > [*] https://lkml.kernel.org/r/324a95ee-b962-acdf-9bd7-b8b23b9fb991@amd.com > > > + } > > + > > entry->eax = g_phys_as | (virt_as << 8); > > entry->edx = 0; > > cpuid_entry_override(entry, CPUID_8000_0008_EBX); > > -- > > 2.31.1 > >