linux-kernel - Re: [PATCH 3/9] x86/hyperv: Mark Hyper-V vp assist page unencrypted in SEV-SNP enlightened guest

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Date:   Tue, 06 Jun 2023 17:49:15 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Tianyu Lan <ltykernel@...il.com>, kys@...rosoft.com,
        haiyangz@...rosoft.com, wei.liu@...nel.org, decui@...rosoft.com,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        daniel.lezcano@...aro.org, arnd@...db.de,
        michael.h.kelley@...rosoft.com
Cc:     Tianyu Lan <tiala@...rosoft.com>, linux-arch@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/9] x86/hyperv: Mark Hyper-V vp assist page unencrypted
 in SEV-SNP enlightened guest

Tianyu Lan <ltykernel@...il.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