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

Maxim Levitsky
2024-11-06T16:13:44Z

On Wed, 2024-11-06 at 09:58 -0600, Mario Limonciello wrote:
> 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.

I am very glad to hear that, thanks!

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

In other words if I understand correctly, there was an errata and to work
it around on the affected CPUs, AMD decided to disable the feature in CPUID,
which is reasonable, but some BIOS vendors forgot to do this.

It all makes sense, thanks again!

Best regards,
Maxim Levitsky

>
> > 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.
>