> > > 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;
> }
> }
> }
