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Message-ID: <4103a70f-cc09-a966-3efa-5ab9273f5c55@gmail.com>
Date:   Tue, 6 Jun 2023 23:22:38 +0800
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Subject: Re: [PATCH 3/9] x86/hyperv: Mark Hyper-V vp assist page unencrypted
 in SEV-SNP enlightened guest
To:     Vitaly Kuznetsov <vkuznets@redhat.com>, kys@microsoft.com,
        haiyangz@microsoft.com, wei.liu@kernel.org, decui@microsoft.com,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com,
        daniel.lezcano@linaro.org, arnd@arndb.de,
        michael.h.kelley@microsoft.com
Cc:     Tianyu Lan <tiala@microsoft.com>, linux-arch@vger.kernel.org,
        linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20230601151624.1757616-1-ltykernel@gmail.com>
 <20230601151624.1757616-4-ltykernel@gmail.com> <873536ksye.fsf@redhat.com>
From:   Tianyu Lan <ltykernel@gmail.com>
In-Reply-To: <873536ksye.fsf@redhat.com>
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On 6/5/2023 8:13 PM, Vitaly Kuznetsov wrote:
>> @@ -113,6 +114,11 @@ static int hv_cpu_init(unsigned int cpu)
>>   
>>   	}
>>   	if (!WARN_ON(!(*hvp))) {
>> +		if (hv_isolation_type_en_snp()) {
>> +			WARN_ON_ONCE(set_memory_decrypted((unsigned long)(*hvp), 1));
>> +			memset(*hvp, 0, PAGE_SIZE);
>> +		}
> Why do we need to set the page as decrypted here and not when we
> allocate the page (a few lines above)?

If Linux root partition boots in the SEV-SNP guest, the page still needs 
to be decrypted.

> And why do we need to clear it
> _after_  we made it decrypted? In case we care about not leaking the
> stale content to the hypervisor, we should've cleared it_before_, but
> the bigger problem I see is that memset() is problemmatic e.g. for KVM
> which uses enlightened VMCS. You put a CPU offline and then back online
> and this path will be taken. Clearing VP assist page will likely brake
> things. (AFAIU SEV-SNP Hyper-V guests don't expose SVM yet so the
> problem is likely theoretical only, but still).
> 

The page will be made dirt by hardware after decrypting operation and so 
memset the page after that.