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

[Vitaly Kuznetsov]

Sean Christopherson <seanjc@xxxxxxxxxx> 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@xxxxxxxxxx/

Hey,

it seems I'm only three months late to the party!

> [2] https://lkml.kernel.org/r/7e3a90c0-75a1-b8fe-dbcf-bda16502ace9@xxxxxxx
> [3] https://lkml.kernel.org/r/20210805105423.412878-1-pbonzini@xxxxxxxxxx
>

This patch helps indeed, thanks! Paolo, any particular reason you
haven't queued it yet?

>>  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 <vkuznets@xxxxxxxxxx>
>> ---
>> - 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@xxxxxxx
>

I see, thanks for the pointer.

>> +		}
>> +
>>  		entry->eax = g_phys_as | (virt_as << 8);
>>  		entry->edx = 0;
>>  		cpuid_entry_override(entry, CPUID_8000_0008_EBX);
>> -- 
>> 2.31.1
>> 
>

-- 
Vitaly