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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Tianyu Lan <ltykernel@gmail.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
Subject: Re: [PATCH 3/9] x86/hyperv: Mark Hyper-V vp assist page unencrypted
 in SEV-SNP enlightened guest
In-Reply-To: <4103a70f-cc09-a966-3efa-5ab9273f5c55@gmail.com>
References: <20230601151624.1757616-1-ltykernel@gmail.com>
 <20230601151624.1757616-4-ltykernel@gmail.com> <873536ksye.fsf@redhat.com>
 <4103a70f-cc09-a966-3efa-5ab9273f5c55@gmail.com>
Date:   Tue, 06 Jun 2023 17:49:15 +0200
Message-ID: <87o7lsk2v8.fsf@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Tianyu Lan <ltykernel@gmail.com> writes:

> 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.
>

I'd suggest we add a flag to indicate that VP assist page was actually
set (on the first invocation of hv_cpu_init() for guest partitions and
all invocations for root partition) and only call
set_memory_decrypted()/memset() then: that would both help with the
potential issue with KVM using enlightened vmcs and avoid the unneeded
hypercall.

-- 
Vitaly