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

[Mario Limonciello]

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@xxxxxxx>
> > > > >
> > > > > 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@xxxxxxxxxxxxxxx
> > > > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=219009
> > > > > Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
> > > > > ---
> > > > >    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.