Re: [RFC PATCH V2 04/18] x86/hyperv: Decrypt hv vp assist page in sev-snp enlightened guest

[Tianyu Lan <ltykernel@gmail.com>]

On 12/13/2022 3:41 AM, Michael Kelley (LINUX) wrote:
>> @@ -228,6 +234,12 @@ static int hv_cpu_die(unsigned int cpu)
>>
>>   	if (hv_vp_assist_page && hv_vp_assist_page[cpu]) {
>>   		union hv_vp_assist_msr_contents msr = { 0 };
>> +
>> +		if (hv_isolation_type_en_snp())
>> +			WARN_ON_ONCE(set_memory_encrypted(
>> +				    (unsigned long)hv_vp_assist_page[cpu],
>> +				    1) != 0);
>> +
> The re-encryption should not be done here (or anywhere else, for
> that matter) since the VP assist pages are never freed.   The Hyper-V
> synthetic MSR pointing to the page gets cleared, but the memory isn't
> freed.  If the CPU should come back online later, the previously allocated
> VP assist page is reused.   The decryption in hv_cpu_init() is done only
> when a new page is allocated to use as a VP assist page.  So just leave
> the page decrypted here, and it will get reused in its decrypted state.
> 
> This handling of the VP assist page is admittedly a bit weird.  But
> it is needed.  See discussion with Vitaly Kuznetsov here:
> https://lore.kernel.org/linux-hyperv/878rkqr7ku.fsf@ovpn-192-136.brq.redhat.com/
> 
Agree. Good point. Will update in the next version.

Thanks.