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

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Message-ID: <ZpFZg-9MTveHfn_4@google.com>
Date: Fri, 12 Jul 2024 09:27:47 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xin3 Li <xin3.li@...el.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>, 
	"tglx@...utronix.de" <tglx@...utronix.de>, "mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>, 
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "x86@...nel.org" <x86@...nel.org>, 
	"hpa@...or.com" <hpa@...or.com>, "shuah@...nel.org" <shuah@...nel.org>, 
	"vkuznets@...hat.com" <vkuznets@...hat.com>, "peterz@...radead.org" <peterz@...radead.org>, 
	Ravi V Shankar <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

On Fri, Jul 12, 2024, Xin3 Li wrote:
> > > > > Switch MSR_IA32_FRED_RSP0 between host and guest in
> > Alternatively, is the desired RSP0 value tracked anywhere other than the MSR?
> 
> Yes, It's simply "(unsigned long)task_stack_page(task) + THREAD_SIZE".
> 
> > 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.

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;
                }
        }
 }