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Date: Wed, 29 May 2024 13:02:06 -0700
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Subject: Re: [PATCH v15 09/20] KVM: SEV: Add support to handle MSR based Page
 State Change VMGEXIT
From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Binbin Wu <binbin.wu@linux.intel.com>, Michael Roth <michael.roth@amd.com>, kvm@vger.kernel.org, 
	linux-coco@lists.linux.dev, linux-mm@kvack.org, linux-crypto@vger.kernel.org, 
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	Brijesh Singh <brijesh.singh@amd.com>, Isaku Yamahata <isaku.yamahata@intel.com>
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On Tue, May 28, 2024, Paolo Bonzini wrote:
> On Mon, May 27, 2024 at 2:26=E2=80=AFPM 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 great=
er
> > > than TDX_MAP_GPA_MAX_LEN. But at that point you're effectively done w=
ith
> > > the entire request and can return to guest, so it actually seems a li=
ttle
> > > more straightforward than the SNP case above. E.g. TDX has a 1:1 mapp=
ing
> > > between TDG_VP_VMCALL_MAP_GPA and KVM_HC_MAP_GPA_RANGE events. (And e=
ven
> > > similar names :))
> > >
> > > So doesn't seem like there's a good reason to expose any of these
> > > throttling details to userspace,
>=20
> 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 swi=
tches.

>  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 attribut=
e,
>  establishing the size that will be processed in a single TDVMCALL.

Is it just interrupts that are problematic for conversions?  I assume so, b=
ecause
I can't think of anything else where telling the guest to retry would be ap=
propriate
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 t=
hat,
KVM could use a much smaller chunk size, e.g. 64KiB (completely made up num=
ber),
and keep processing the TDVMCALL as long as there is no interrupt pending.
Hopefully that would obviate the need for a tunable.