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

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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;
            }
    }
  

}