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Date:   Mon, 14 Nov 2022 08:23:28 -0800
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Subject: Re: [PATCH v2 02/12] x86/ioapic: Gate decrypted mapping on
 cc_platform_has() attribute
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To:     "Michael Kelley (LINUX)" <mikelley@microsoft.com>,
        "hpa@zytor.com" <hpa@zytor.com>, KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
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        "kirill.shutemov@linux.intel.com" <kirill.shutemov@linux.intel.com>,
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References: <1668147701-4583-1-git-send-email-mikelley@microsoft.com>
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From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <BYAPR21MB168860D4D19F088CB41E7548D7039@BYAPR21MB1688.namprd21.prod.outlook.com>
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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.