linux-kernel - Re: [RFC PATCH 0/8] KVM: Prepopulate guest memory API

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Message-ID: <Ze-XW-EbT9vXaagC@google.com>
Date: Mon, 11 Mar 2024 16:44:27 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Michael Roth <michael.roth@....com>
Cc: isaku.yamahata@...el.com, kvm@...r.kernel.org, isaku.yamahata@...il.com, 
	linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, 
	David Matlack <dmatlack@...gle.com>, Federico Parola <federico.parola@...ito.it>
Subject: Re: [RFC PATCH 0/8] KVM: Prepopulate guest memory API

On Sun, Mar 10, 2024, Michael Roth wrote:
> On Fri, Mar 01, 2024 at 09:28:42AM -0800, isaku.yamahata@...el.com wrote:
> >   struct kvm_sev_launch_update_data {
> >         __u64 uaddr;
> >         __u32 len;
> >   };
> > 
> > - TDX and measurement
> >   The TDX correspondence is TDH.MEM.PAGE.ADD and TDH.MR.EXTEND.  TDH.MEM.EXTEND
> >   extends its measurement by the page contents.
> >   Option 1. Add an additional flag like KVM_MEMORY_MAPPING_FLAG_EXTEND to issue
> >             TDH.MEM.EXTEND
> >   Option 2. Don't handle extend. Let TDX vendor specific API
> >             KVM_EMMORY_ENCRYPT_OP to handle it with the subcommand like
> >             KVM_TDX_EXTEND_MEMORY.
> 
> For SNP this happens unconditionally via SNP_LAUNCH_UPDATE, and with some
> additional measurements via SNP_LAUNCH_FINISH, and down the road when live
> migration support is added that flow will be a bit different. So
> personally I think it's better to leave separate for now.

+1.  The only reason to do EXTEND at the same time as PAGE.ADD would be to
optimize setups that want the measurement to be extended with the contents of a
page immediately after the measurement is extended with the mapping metadata for
said page.  And AFAIK, the only reason to prefer that approach is for backwards
compatibility, which is not a concern for KVM.  I suppose maaaybe some memory
locality performance benefits, but that seems like a stretch.

<time passes>

And I think this whole conversation is moot, because I don't think there's a need
to do PAGE.ADD during KVM_MAP_MEMORY[*].  If KVM_MAP_MEMORY does only the SEPT.ADD
side of things, then both @source (PAGE.ADD) and the EXTEND flag go away.

> But I'd be hesitant to bake more requirements into this pre-mapping
> interface, it feels like we're already overloading it as is.

Agreed.  After being able to think more about this ioctl(), I think KVM_MAP_MEMORY
should be as "pure" of a mapping operation as we can make it.  It'd be a little
weird that using KVM_MAP_MEMORY is required for TDX VMs, but not other VMs.  But
that's really just a reflection of S-EPT, so it's arguably not even a bad thing.

[*] https://lore.kernel.org/all/Ze-TJh0BBOWm9spT@google.com