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

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Message-ID: <407ab4f1-eac2-21ce-95d7-e861a1da73c8@gmail.com>
Date:   Tue, 13 Dec 2022 23:21:44 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
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        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "jgross@...e.com" <jgross@...e.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
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Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
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Subject: Re: [RFC PATCH V2 04/18] x86/hyperv: Decrypt hv vp assist page in
 sev-snp enlightened guest

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.