Re: [PATCH] x86/CPU/AMD: Clear virtualized VMLOAD/VMSAVE on Zen4 client

Mario Limonciello
2024-11-06T15:58:54Z

On 11/6/2024 09:48, Maxim Levitsky wrote:
> On Wed, 2024-11-06 at 07:15 -0800, Sean Christopherson wrote:
>> On Wed, Nov 06, 2024, Mario Limonciello wrote:
>>> On 11/6/2024 09:03, Sean Christopherson wrote:
>>>> +KVM, given that this quite obviously affects KVM...
>>>>
>>>> On Tue, Nov 05, 2024, Mario Limonciello wrote:
>>>>> From: Mario Limonciello <mario.limonciello@amd.com>
>>>>>
>>>>> A number of Zen4 client SoCs advertise the ability to use virtualized
>>>>> VMLOAD/VMSAVE, but using these instructions is reported to be a cause
>>>>> of a random host reboot.
>>>>>
>>>>> These instructions aren't intended to be advertised on Zen4 client
>>>>> so clear the capability.
>>>>>
>>>>> Cc: stable@vger.kernel.org
>>>>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=219009
>>>>> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
>>>>> ---
>>>>> arch/x86/kernel/cpu/amd.c | 11 +++++++++++
>>>>> 1 file changed, 11 insertions(+)
>>>>>
>>>>> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
>>>>> index 015971adadfc7..ecd42c2b3242e 100644
>>>>> --- a/arch/x86/kernel/cpu/amd.c
>>>>> +++ b/arch/x86/kernel/cpu/amd.c
>>>>> @@ -924,6 +924,17 @@ static void init_amd_zen4(struct cpuinfo_x86 *c)
>>>>> {
>>>>> if (!cpu_has(c, X86_FEATURE_HYPERVISOR))
>>>>> msr_set_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_SHARED_BTB_FIX_BIT);
>>>>> +
>>>>> + /*
>>>>> + * These Zen4 SoCs advertise support for virtualized VMLOAD/VMSAVE
>>>>> + * in some BIOS versions but they can lead to random host reboots.
>>>>
>>>> Uh, CPU bug? Erratum?
>>>
>>> BIOS bug. Those shouldn't have been advertised.
>
> Hi!
>
> My question is, why would AMD drop support intentionally for VLS on client machines?
>
> I understand that there might be a errata, and I don't object disabling the
> feature because of this.
>
> But hearing that 'These instructions aren't intended to be advertised' means that
> AMD intends to stop supporting virtualization on client systems or at least partially
> do so.

Don't read into it too far. It's just a BIOS problem with those
instructions "specifically" on the processors indicated here. Other
processors (for example Zen 5 client processors) do correctly advertise
support where applicable.

When they launched those bits weren't supposed to be set to indicate
support, but BIOS did set them.

>
> That worries me. So far AMD was much better that Intel supporting most of the
> features across all of the systems which is very helpful in various scenarios,
> and this is very appreciated by the community.
>
> Speaking strictly personally here, as a AMD fan.
>
> Best regards,> Maxim Levitsky
>
>
>>
>> Why not? "but they can lead to random host reboots" is a description of the
>> symptom, not an explanation for why KVM is unable to use a feature that is
>> apparently support by the CPU.
>>
>> And if the CPU doesn't actually support virtualized VMLOAD/VMSAVE, then this is
>> a much bigger problem, because it means KVM is effectively giving the guest read
>> and write access to all of host memory.
>>
>
>

I'm gathering that what supported means to you and what it means to me
are different things. "Architecturally" the instructions for
virtualized VMLOAD/VMSAVE exist. There are problems with them on these
processors, and for that reason the BIOS was not supposed to set those
bits but it did.