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

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Message-ID: <ZyuIINwBdiztWhi3@google.com>
Date: Wed, 6 Nov 2024 07:15:44 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Mario Limonciello <superm1@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, 
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>, Nikolay Borisov <nik.borisov@...e.com>, 
	Tom Lendacky <thomas.lendacky@....com>, Brijesh Singh <brijesh.singh@....com>, 
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org>, 
	Mario Limonciello <mario.limonciello@....com>, kvm@...r.kernel.org
Subject: Re: [PATCH] x86/CPU/AMD: Clear virtualized VMLOAD/VMSAVE on Zen4 client

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