linux-kernel - Re: [PATCH 1/2] x86/sev/vc: fix efi runtime instruction emulation

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Message-ID: <77hywpberfkulac3q3hpupdmdpw2xbmlvzin4ks7xypikravkj@xjpi7gqscs6a>
Date: Wed, 28 May 2025 09:38:24 +0200
From: Gerd Hoffmann <kraxel@...hat.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, 
	kvm@...r.kernel.org, linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/sev/vc: fix efi runtime instruction emulation

On Tue, May 27, 2025 at 06:21:51PM +0200, Borislav Petkov wrote:
> On Tue, May 27, 2025 at 04:45:44PM +0200, Gerd Hoffmann wrote:
> > In case efi_mm is active go use the userspace instruction decoder which
> > supports fetching instructions from active_mm.  This is needed to make
> > instruction emulation work for EFI runtime code, so it can use cpuid
> > and rdmsr.
> > 
> > Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
> > ---
> >  arch/x86/coco/sev/core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Can you pls explain what the use cases for this and your next patch are?

Use case is coconut-svsm providing an uefi variable store and edk2
runtime code doing svsm protocol calls to send requests to the svsm
variable store.  edk2 needs a caa page mapping and a working rdmsr
instruction for that.

Another less critical but useful case is edk2 debug logging to qemu
debugcon port.  That needs a working cpuid instruction because edk2
uses that to figure whenever sev is active and adapt ioport access
accordingly.

> We'd like to add them to our test pile.

That is a bit difficult right now because there are a number of pieces
which need to fall into place before this is easily testable.  You need:

 * host kernel with vmplanes patch series (for snp vmpl support).
 * coconut svsm with uefi variable store patches.
 * edk2 patches so it talks to svsm for variable access.
 * igvm support patches for qemu.

Hope I didn't forgot something ...

take care,
  Gerd