linux-kernel - Re: [PATCH RFC] KVM: SVM: reduce guest MAXPHYADDR by one in case C-bit is a physical bit

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Message-ID: <874k9ey3tz.fsf@vitty.brq.redhat.com>
Date:   Mon, 18 Oct 2021 13:23:04 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Matlack <dmatlack@...gle.com>,
        linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH RFC] KVM: SVM: reduce guest MAXPHYADDR by one in case
 C-bit is a physical bit

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> Sean Christopherson <seanjc@...gle.com> writes:
>
>> 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/
>
> Hey,
>
> it seems I'm only three months late to the party!
>
>> [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
>>
>
> This patch helps indeed

FWIW, 'access_tracking_perf_test' remains broken even after the patch is
applied:

# ./access_tracking_perf_test 
Testing guest mode: PA-bits:ANY, VA-bits:48,  4K pages
guest physical test memory offset: 0xfffcbffff000

Populating memory             : 3.858448918s
Writing to populated memory   : 0.937319626s
Reading from populated memory : 0.003073207s
==== Test Assertion Failure ====
  lib/kvm_util.c:1382: false
  pid=6422 tid=6425 errno=4 - Interrupted system call
     1	0x000000000040667d: addr_gpa2hva at kvm_util.c:1382
     2	 (inlined by) addr_gpa2hva at kvm_util.c:1376
     3	 (inlined by) addr_gva2hva at kvm_util.c:2245
     4	0x0000000000402909: lookup_pfn at access_tracking_perf_test.c:98
     5	 (inlined by) mark_vcpu_memory_idle at access_tracking_perf_test.c:152
     6	 (inlined by) vcpu_thread_main at access_tracking_perf_test.c:232
     7	0x00007fd02d1cb431: ?? ??:0
     8	0x00007fd02d0f9912: ?? ??:0
  No vm physical memory at 0xfcbffff000

(and my cpuid hack reducing guest physical address space by half doesn't
seem to help either)

-- 
Vitaly