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From: Paolo Bonzini <pbonzini@redhat.com>
Date: Tue, 28 May 2024 12:39:36 +0200
Message-ID: <CABgObfajCDkbDbK6-QyZABGTh=5rmE5q3ifvHfZD1A2Z+u0v3A@mail.gmail.com>
Subject: Re: [PATCH v15 09/20] KVM: SEV: Add support to handle MSR based Page
 State Change VMGEXIT
To: Binbin Wu <binbin.wu@linux.intel.com>
Cc: Michael Roth <michael.roth@amd.com>, kvm@vger.kernel.org, linux-coco@lists.linux.dev, 
	linux-mm@kvack.org, linux-crypto@vger.kernel.org, x86@kernel.org, 
	linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com, 
	jroedel@suse.de, thomas.lendacky@amd.com, hpa@zytor.com, ardb@kernel.org, 
	seanjc@google.com, vkuznets@redhat.com, jmattson@google.com, luto@kernel.org, 
	dave.hansen@linux.intel.com, slp@redhat.com, pgonda@google.com, 
	peterz@infradead.org, srinivas.pandruvada@linux.intel.com, 
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	sathyanarayanan.kuppuswamy@linux.intel.com, alpergun@google.com, 
	jarkko@kernel.org, ashish.kalra@amd.com, nikunj.dadhania@amd.com, 
	pankaj.gupta@amd.com, liam.merwick@oracle.com, 
	Brijesh Singh <brijesh.singh@amd.com>, "Yamahata, Isaku" <isaku.yamahata@intel.com>
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On Mon, May 27, 2024 at 2:26=E2=80=AFPM Binbin Wu <binbin.wu@linux.intel.co=
m> wrote:
> > It seems like TDX should be able to do something similar by limiting th=
e
> > 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 wit=
h
> > the entire request and can return to guest, so it actually seems a litt=
le
> > more straightforward than the SNP case above. E.g. TDX has a 1:1 mappin=
g
> > between TDG_VP_VMCALL_MAP_GPA and KVM_HC_MAP_GPA_RANGE events. (And eve=
n
> > 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, 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.

Paolo

>
> The reasons I want to put the throttling in userspace are:
> 1. Hardcode the TDX_MAP_GPA_MAX_LEN in kernel may not be preferred.
> 2. The throttling thing doesn't need to be TDX specific, it can be
> generic in userspace.
>
> I think we can set a reasonable value in userspace, so that for SNP, it
> doesn't trigger the throttling since the large request will be split to
> multiple userspace requests.
>
>
> > in which case existing
> > KVM_HC_MAP_GPA_RANGE interface seems like it should be sufficient.
> >
> > -Mike
> >
> >>
> >>>> For TDX, it may also want to use KVM_HC_MAP_GPA_RANGE hypercall  to
> >>>> userspace via KVM_EXIT_HYPERCALL.
> >>> Yes, definitely.
> >>>
> >>> Paolo
> >>>
>