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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id x7sm5292267pfj.28.2021.10.15.08.24.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 15 Oct 2021 08:24:32 -0700 (PDT) Date: Fri, 15 Oct 2021 15:24:28 +0000 From: Sean Christopherson To: Vitaly Kuznetsov Cc: kvm@vger.kernel.org, Paolo Bonzini , Wanpeng Li , Jim Mattson , Tom Lendacky , David Matlack , linux-kernel@vger.kernel.org Subject: Re: [PATCH RFC] KVM: SVM: reduce guest MAXPHYADDR by one in case C-bit is a physical bit Message-ID: References: <20211015150524.2030966-1-vkuznets@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211015150524.2030966-1-vkuznets@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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]. [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 >