RE: [PATCH v2 02/12] x86/ioapic: Gate decrypted mapping on cc_platform_has() attribute

[Michael Kelley (LINUX)]

From: Dave Hansen <dave.hansen@xxxxxxxxx> Sent: Friday, November 11, 2022 4:22 PM
> 
> On 11/10/22 22:21, Michael Kelley wrote:
> >  	 * Ensure fixmaps for IOAPIC MMIO respect memory encryption pgprot
> >  	 * bits, just like normal ioremap():
> >  	 */
> > -	flags = pgprot_decrypted(flags);
> > +	if (!cc_platform_has(CC_ATTR_HAS_PARAVISOR))
> > +		flags = pgprot_decrypted(flags);
> 
> This begs the question whether *all* paravisors will want to avoid a
> decrypted ioapic mapping.  Is this _fundamental_ to paravisors, or it is
> an implementation detail of this _individual_ paravisor?

Hard to say.  The paravisor that Hyper-V provides for use with the vTOM
option in a SEV SNP VM is the only paravisor I've seen.  At least as defined
by Hyper-V and AMD SNP Virtual Machine Privilege Levels (VMPLs), the
paravisor resides within the VM trust boundary.  Anything that a paravisor
emulates would be in the "private" (i.e., encrypted) memory so it can be
accessed by both the guest OS and the paravisor.  But nothing fundamental
says that IOAPIC emulation *must* be done in the paravisor.

I originally though about naming this attribute HAS_EMULATED_IOAPIC, but
that felt a bit narrow as other emulated hardware might need similar treatment
in the future, at least with the Hyper-V and AMD SEV SNP vTOM paravisor.

Net, we currently have N=1 for paravisors, and we won't know what the more
generalized case looks like until N >= 2.  If/when that happens, additional logic
might be needed here, and the name of this attribute might need adjustment
to support broader usage.  But if there's consensus on a different name now,
or on the narrower HAS_EMULATED_IOAPIC name, it doesn’t really matter
to me.

Michael