linux-kernel - RE: [PATCH v2 02/12] x86/ioapic: Gate decrypted mapping on cc_platform

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Message-ID: <BYAPR21MB1688D90D14EBD1363F340A57D7059@BYAPR21MB1688.namprd21.prod.outlook.com>
Date:   Mon, 14 Nov 2022 16:54:38 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "hpa@...or.com" <hpa@...or.com>, KY Srinivasan <kys@...rosoft.com>,
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Subject: RE: [PATCH v2 02/12] x86/ioapic: Gate decrypted mapping on
 cc_platform_has() attribute

From: Dave Hansen <dave.hansen@...el.com> Sent: Monday, November 14, 2022 8:23 AM
> 
> On 11/11/22 20:48, Michael Kelley (LINUX) wrote:
> > From: Dave Hansen <dave.hansen@...el.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.

I'm good with that.  I'll use HAS_EMULATED_IOAPIC in v3.

Michael