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

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Date: Wed, 29 May 2024 13:02:06 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Binbin Wu <binbin.wu@...ux.intel.com>, Michael Roth <michael.roth@....com>, kvm@...r.kernel.org, 
	linux-coco@...ts.linux.dev, linux-mm@...ck.org, linux-crypto@...r.kernel.org, 
	x86@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de, 
	mingo@...hat.com, jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com, 
	ardb@...nel.org, vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org, 
	dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com, 
	peterz@...radead.org, srinivas.pandruvada@...ux.intel.com, 
	rientjes@...gle.com, dovmurik@...ux.ibm.com, tobin@....com, bp@...en8.de, 
	vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com, tony.luck@...el.com, 
	sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com, 
	jarkko@...nel.org, ashish.kalra@....com, nikunj.dadhania@....com, 
	pankaj.gupta@....com, liam.merwick@...cle.com, 
	Brijesh Singh <brijesh.singh@....com>, Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [PATCH v15 09/20] KVM: SEV: Add support to handle MSR based Page
 State Change VMGEXIT

On Tue, May 28, 2024, Paolo Bonzini wrote:
> On Mon, May 27, 2024 at 2:26 PM Binbin Wu <binbin.wu@...ux.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.