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

Dave Hansen
2022-11-14T16:23:44Z

On 11/11/22 20:48, Michael Kelley (LINUX) wrote:
> From: Dave Hansen <dave.hansen@intel.com> 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.

Please just make this check more specific. Either make this a specific
Hyper-V+SVM check, or rename it HAS_EMULATED_IOAPIC, like you were
thinking. If paravisors catch on and we end up with ten more of these
things across five different paravisors and see a pattern, *then* a
paravisor-specific one makes sense.