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

[Sean Christopherson <seanjc@google.com>]

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.

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.