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

[H. Peter Anvin]

On July 12, 2024 8:12:51 AM PDT, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>On Wed, Jul 10, 2024, Xin3 Li wrote:
>> > On Wed, Feb 07, 2024, Xin Li wrote:
>> > > Switch MSR_IA32_FRED_RSP0 between host and guest in
>> > > vmx_prepare_switch_to_{host,guest}().
>> > >
>> > > MSR_IA32_FRED_RSP0 is used during ring 3 event delivery only, thus
>> > > KVM, running on ring 0, can run safely with guest FRED RSP0, i.e., no
>> > > need to switch between host/guest FRED RSP0 during VM entry and exit.
>> > >
>> > > KVM should switch to host FRED RSP0 before returning to user level,
>> > > and switch to guest FRED RSP0 before entering guest mode.
>> > 
>> > Heh, if only KVM had a framework that was specifically designed for context
>> > switching MSRs on return to userspace.  Translation: please use the
>> > user_return_msr() APIs.
>> 
>> IIUC the user return MSR framework works for MSRs that are per CPU
>> constants, but like MSR_KERNEL_GS_BASE, MSR_IA32_FRED_RSP0 is a per
>> *task* constant, thus we can't use it.
>
>Ah, in that case, the changelog is very misleading and needs to be fixed.
>Alternatively, is the desired RSP0 value tracked anywhere other than the MSR?
>E.g. if it's somewhere in task_struct, then kvm_on_user_return() would restore
>the current task's desired RSP0.  Even if we don't get fancy, avoiding the RDMSR
>to get the current task's value would be nice.

Hm, perhaps the right thing to do is to always invoke this function before a context switch happens if that happens before return to user space?