On Thu, Feb 23, 2023, Michael Kelley (LINUX) wrote:
> From: Dave Hansen <dave.hansen@xxxxxxxxx> 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.
