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

[Sean Christopherson <seanjc@xxxxxxxxxx>]

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;
                }
        }
 }