Re: [PATCH v15 09/20] KVM: SEV: Add support to handle MSR based Page State Change VMGEXIT

[Sean Christopherson <seanjc@google.com>]

On Tue, May 28, 2024, Paolo Bonzini wrote:
> On Mon, May 27, 2024 at 2:26 PM Binbin Wu <binbin.wu@linux.intel.com> wrote:
> > > It seems like TDX should be able to do something similar by limiting the
> > > size of each KVM_HC_MAP_GPA_RANGE to TDX_MAP_GPA_MAX_LEN, and then
> > > returning TDG_VP_VMCALL_RETRY to guest if the original size was greater
> > > than TDX_MAP_GPA_MAX_LEN. But at that point you're effectively done with
> > > the entire request and can return to guest, so it actually seems a little
> > > more straightforward than the SNP case above. E.g. TDX has a 1:1 mapping
> > > between TDG_VP_VMCALL_MAP_GPA and KVM_HC_MAP_GPA_RANGE events. (And even
> > > similar names :))
> > >
> > > So doesn't seem like there's a good reason to expose any of these
> > > throttling details to userspace,
> 
> I think userspace should never be worried about throttling. I would
> say it's up to the guest to split the GPA into multiple ranges,

I agree in principle, but in practice I can understand not wanting to split up
the conversion in the guest due to the additional overhead of the world switches.

>  but that's not how arch/x86/coco/tdx/tdx.c is implemented so instead we can
>  do the split in KVM instead. It can be a module parameter or VM attribute,
>  establishing the size that will be processed in a single TDVMCALL.

Is it just interrupts that are problematic for conversions?  I assume so, because
I can't think of anything else where telling the guest to retry would be appropriate
and useful.

If so, KVM shouldn't need to unconditionally restrict the size for a single
TDVMCALL, KVM just needs to ensure interrupts are handled soonish.  To do that,
KVM could use a much smaller chunk size, e.g. 64KiB (completely made up number),
and keep processing the TDVMCALL as long as there is no interrupt pending.
Hopefully that would obviate the need for a tunable.