linux-kernel - RE: [PATCH v2 09/25] KVM: VMX: Switch FRED RSP0 between host and guest

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Message-ID: <SA1PR11MB67349D5D7B0E26A49B8A1112A8A62@SA1PR11MB6734.namprd11.prod.outlook.com>
Date: Fri, 12 Jul 2024 17:17:37 +0000
From: "Li, Xin3" <xin3.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>, "linux-kselftest@...r.kernel.org"
	<linux-kselftest@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "corbet@....net" <corbet@....net>,
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	"peterz@...radead.org" <peterz@...radead.org>, "Shankar, Ravi V"
	<ravi.v.shankar@...el.com>, "xin@...or.com" <xin@...or.com>
Subject: RE: [PATCH v2 09/25] KVM: VMX: Switch FRED RSP0 between host and
 guest

> > > E.g. if it's somewhere in task_struct, then kvm_on_user_return()
> > > would restore the current task's desired RSP0.
> >
> > So you're suggesting to extend the framework to allow per task constants?
> 
> Yeah, or more likely, special case MSR_IA32_FRED_RSP0.  If KVM didn't already
> have the user return framework, I wouldn't suggest this as I doubt avoiding
> WRMSR when switching between vCPU tasks will be very meaningful, but it's
> easy to handle FRED_RSP0, so why not.

Great, I will take the patch.

It looks to me that this also works for KERNEL GS BASE MSR, no?

> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index
> 1783986d8626..ebecb205e5de 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -352,6 +352,7 @@ static void kvm_on_user_return(struct
> user_return_notifier *urn)
>                 = container_of(urn, struct kvm_user_return_msrs, urn);
>         struct kvm_user_return_msr_values *values;
>         unsigned long flags;
> +       u64 host_val;
> 
>         /*
>          * Disabling irqs at this point since the following code could be @@ -365,9
> +366,15 @@ static void kvm_on_user_return(struct user_return_notifier *urn)
>         local_irq_restore(flags);
>         for (slot = 0; slot < kvm_nr_uret_msrs; ++slot) {
>                 values = &msrs->values[slot];
> -               if (values->host != values->curr) {
> -                       wrmsrl(kvm_uret_msrs_list[slot], values->host);
> -                       values->curr = values->host;
> +
> +               if (kvm_uret_msrs_list[slot] == MSR_IA32_FRED_RSP0)
> +                       host_val = get_current_fred_rsp0();
> +               else
> +                       host_val = values->host;
> +
> +               if (host_val != values->curr) {
> +                       wrmsrl(kvm_uret_msrs_list[slot], host_val);
> +                       values->curr = host_val;
>                 }
>         }
>  }