linux-kernel - Re: [PATCH 1/2] KVM: nVMX: fix CR4_READ_SHADOW when L0 updates CR4 during a signal

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Date: Thu, 18 Apr 2024 11:28:06 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Thomas Prescher <thomas.prescher@...erus-technology.de>
Cc: "mingo@...hat.com" <mingo@...hat.com>, "x86@...nel.org" <x86@...nel.org>, 
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "hpa@...or.com" <hpa@...or.com>, 
	Julian Stecklina <julian.stecklina@...erus-technology.de>, 
	"tglx@...utronix.de" <tglx@...utronix.de>, "bp@...en8.de" <bp@...en8.de>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] KVM: nVMX: fix CR4_READ_SHADOW when L0 updates CR4
 during a signal

On Thu, Apr 18, 2024, Thomas Prescher wrote:
> On Wed, 2024-04-17 at 09:28 -0700, Sean Christopherson wrote:
> > On Wed, Apr 17, 2024, Sean Christopherson wrote:
> > > On Wed, Apr 17, 2024, Thomas Prescher wrote:
> > > > On Tue, 2024-04-16 at 11:07 -0700, Sean Christopherson wrote:
> > > > > Hur dur, I forgot that KVM provides a "guest_mode" stat. 
> > > > > Userspace can do
> > > > > KVM_GET_STATS_FD on the vCPU FD to get a file handle to the
> > > > > binary stats,
> > > > > and then you wouldn't need to call back into KVM just to query
> > > > > guest_mode.
> > > > > 
> > > > > Ah, and I also forgot that we have kvm_run.flags, so adding
> > > > > KVM_RUN_X86_GUEST_MODE would also be trivial (I almost
> > > > > suggested it
> > > > > earlier, but didn't want to add a new field to kvm_run without
> > > > > a very good
> > > > > reason).
> > > > 
> > > > Thanks for the pointers. This is really helpful.
> > > > 
> > > > I tried the "guest_mode" stat as you suggested and it solves the
> > > > immediate issue we have with VirtualBox/KVM.
> > > 
> > > Note, 
> > 
> > Gah, got distracted.  I was going to say that we should add
> > KVM_RUN_X86_GUEST_MODE,
> > because stats aren't guaranteed ABI[*], i.e. relying on guest_mode
> > could prove
> > problematic in the long run (though that's unlikely).
> > 
> > [*]
> > https://lore.kernel.org/all/CABgObfZ4kqaXLaOAOj4aGB5GAe9GxOmJmOP+7kdke6OqA35HzA@mail.gmail.com
> 
> Allright. I will propose a patch that sets the KVM_RUN_X86_GUEST_MODE
> flag in the next couple of days. Do we also need a new capability for
> this flag so userland can query whether this field is actually updated
> by KVM?

Hmm, yeah, I don't see any way around that.