linux-kernel - Re: [PATCH 4/9] drivers: hv: Mark shared pages unencrypted in SEV-SNP enlightened guest

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Message-ID: <2803e5d6-58bc-57f1-0721-226333238883@gmail.com>
Date:   Wed, 7 Jun 2023 16:16:03 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.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 4/9] drivers: hv: Mark shared pages unencrypted in SEV-SNP
 enlightened guest

On 6/5/2023 8:54 PM, Vitaly Kuznetsov wrote:
>> @@ -402,7 +417,14 @@ int hv_common_cpu_die(unsigned int cpu)
>>   
>>   	local_irq_restore(flags);
>>   
>> -	kfree(mem);
>> +	if (hv_isolation_type_en_snp()) {
>> +		ret = set_memory_encrypted((unsigned long)mem, pgcount);
>> +		if (ret)
>> +			pr_warn("Hyper-V: Failed to encrypt input arg on cpu%d: %d\n",
>> +				cpu, ret);
>> +		/* It's unsafe to free 'mem'. */
>> +		return 0;
> Why is it unsafe to free 'mem' if ret == 0? Also, why don't we want to
> proparate non-zero 'ret' from here to fail CPU offlining?
> 

Based on Michael's patch the mem will not be freed during cpu offline.
https://lwn.net/ml/linux-kernel/87cz2j5zrc.fsf@redhat.com/
So I think it's unnessary to encrypt the mem again here.