linux-kernel - Re: [RFC PATCH 6/8] KVM: x86: Implement kvm_arch_{, pre_}vcpu_map

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Date: Wed, 6 Mar 2024 17:51:51 -0800
From: Isaku Yamahata <isaku.yamahata@...ux.intel.com>
To: David Matlack <dmatlack@...gle.com>
Cc: isaku.yamahata@...el.com, kvm@...r.kernel.org, isaku.yamahata@...il.com,
	linux-kernel@...r.kernel.org,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Michael Roth <michael.roth@....com>,
	Federico Parola <federico.parola@...ito.it>,
	isaku.yamahata@...ux.intel.com
Subject: Re: [RFC PATCH 6/8] KVM: x86: Implement kvm_arch_{,
 pre_}vcpu_map_memory()

On Wed, Mar 06, 2024 at 04:36:25PM -0800,
David Matlack <dmatlack@...gle.com> wrote:

> On Wed, Mar 6, 2024 at 4:31 PM David Matlack <dmatlack@...gle.com> wrote:
> >
> > On 2024-03-01 09:28 AM, isaku.yamahata@...el.com wrote:
> > >
> > > +     if (IS_ALIGNED(mapping->base_gfn, KVM_PAGES_PER_HPAGE(PG_LEVEL_1G)) &&
> > > +         mapping->nr_pages >= KVM_PAGES_PER_HPAGE(PG_LEVEL_1G))
> > > +             max_level = PG_LEVEL_1G;
> > > +     else if (IS_ALIGNED(mapping->base_gfn, KVM_PAGES_PER_HPAGE(PG_LEVEL_2M)) &&
> > > +              mapping->nr_pages >= KVM_PAGES_PER_HPAGE(PG_LEVEL_2M))
> > > +             max_level = PG_LEVEL_2M;
> > > +     else
> > > +             max_level = PG_LEVEL_4K;
> >
> > Is there a requirement that KVM must not map memory outside of the
> > requested region?
> 
> And if so, what if the requested region is already mapped with a larger page?

Yes. We'd like to map exact gpa range for SNP or TDX case. We don't want to map
zero at around range.  For SNP or TDX, we map page to GPA, it's one time
operation.  It updates measurement.

Say, we'd like to populate GPA1 and GPA2 with initial guest memory image.  And
they are within same 2M range.  Map GPA1 first. If GPA2 is also mapped with zero
with 2M page, the following mapping of GPA2 fails.  Even if mapping of GPA2
succeeds, measurement may be updated when mapping GPA1. 

It's user space VMM responsibility to map GPA range only once at most for SNP or
TDX.  Is this too strict requirement for default VM use case to mitigate KVM
page fault at guest boot up?  If so, what about a flag like EXACT_MAPPING or
something?
-- 
Isaku Yamahata <isaku.yamahata@...ux.intel.com>