RE: [PATCH v5 06/14] x86/ioremap: Support hypervisor specified range to map as encrypted

From: "Michael Kelley (LINUX)" <mikelley@microsoft.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>,
	Borislav Petkov <bp@alien8.de>, "hpa@zytor.com" <hpa@zytor.com>,
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Subject: RE: [PATCH v5 06/14] x86/ioremap: Support hypervisor specified range to map as encrypted
Date: Thu, 23 Feb 2023 21:15:54 +0000	[thread overview]
Message-ID: <BYAPR21MB1688452212CF8E1B97D8826DD7AB9@BYAPR21MB1688.namprd21.prod.outlook.com> (raw)
In-Reply-To: <Y/fVoc4C5BNI+i7l@google.com>

From: Sean Christopherson <seanjc@google.com> Sent: Thursday, February 23, 2023 1:08 PM
> 
> On Thu, Feb 23, 2023, Michael Kelley (LINUX) wrote:
> > From: Dave Hansen <dave.hansen@intel.com> Sent: Thursday, February 23, 2023
> 12:42 PM
> > >
> > > On 2/23/23 12:26, Dave Hansen wrote:
> > > >> +       if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) {
> > > >> +               /*
> > > >> +               * Ensure fixmaps for IOAPIC MMIO respect memory encryption pgprot
> > > >> +               * bits, just like normal ioremap():
> > > >> +               */
> > > >> +               if (x86_platform.hyper.is_private_mmio(phys))
> > > >> +                       flags = pgprot_encrypted(flags);
> > > >> +               else
> > > >> +                       flags = pgprot_decrypted(flags);
> > > >> +       }
> > > ...
> > > > It does seem a bit odd that there's a new CC_ATTR_GUEST_MEM_ENCRYPT
> > > > check wrapping this whole thing.  I guess the trip through
> > > > pgprot_decrypted() is harmless on normal platforms, though.
> > >
> > > Yeah, that's _really_ odd.  Sean, were you trying to optimize away the
> > > indirect call or something?
> 
> No, my thought was simply to require platforms that support GUEST_MEM_ENCRYPT
> to
> implement x86_platform.hyper.is_private_mmio, e.g. to avoid having to check if
> is_private_mmio is NULL, to explicit document that non-Hyper-V encrypted guests
> don't (yet) support private MMIO, and to add a bit of documentation around the
> {de,en}crypted logic.
> 
> > > I would just expect the Hyper-V/vTOM code to leave
> > > x86_platform.hyper.is_private_mmio alone unless it *knows* the platform has
> > > private MMIO *and* CC_ATTR_GUEST_MEM_ENCRYPT.
> >
> > Agreed.
> >
> > >
> > > Is there ever a case where CC_ATTR_GUEST_MEM_ENCRYPT==0 and he
> > > Hyper-V/vTOM code would need to set x86_platform.hyper.is_private_mmio?
> >
> > There's no such case.
> >
> > I agree that gating with CC_ATTR_GUEST_MEM_ENCRYPT isn't really necessary.
> > Current upstream code always does the pgprot_decrypted(), and as you said,
> > that's a no-op on platforms with no memory encryption.
> 
> Right, but since is_private_mmio can be NULL, unless I'm missing something we'll
> need an extra check no matter what, i.e. the alternative would be
> 
> 	if (x86_platform.hyper.is_private_mmio &&
> 	    x86_platform.hyper.is_private_mmio(phys))
> 		flags = pgprot_encrypted(flags);
> 	else
> 		flags = pgprot_decrypted(flags);
> 
> I have no objection to that approach.  It does have the advantage of not needing
> an indirect call for encrypted guests that don't support private MMIO, though
> I can't imagine this code is performance sensitive.

Or statically set a default stub function for is_private_mmio() that returns "false".
Then there's no need to check for NULL, and only platforms that want to use it
have to code anything.  Several other entries in x86_platform have such defaults.

Michael