Re: [PATCH 05/13] HV: Add Write/Read MSR registers via ghcb page

From: Tianyu Lan <ltykernel@gmail.com>
To: Joerg Roedel <joro@8bytes.org>
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Subject: Re: [PATCH 05/13] HV: Add Write/Read MSR registers via ghcb page
Date: Mon, 2 Aug 2021 21:18:15 +0800	[thread overview]
Message-ID: <988f20e4-821f-b493-c25d-ca9107a6e891@gmail.com> (raw)
In-Reply-To: <YQfk9G+k0Tj8ihyu@8bytes.org>

On 8/2/2021 8:28 PM, Joerg Roedel wrote:
> On Wed, Jul 28, 2021 at 10:52:20AM -0400, Tianyu Lan wrote:
>> +void hv_ghcb_msr_write(u64 msr, u64 value)
>> +{
>> +	union hv_ghcb *hv_ghcb;
>> +	void **ghcb_base;
>> +	unsigned long flags;
>> +
>> +	if (!ms_hyperv.ghcb_base)
>> +		return;
>> +
>> +	WARN_ON(in_nmi());
>> +
>> +	local_irq_save(flags);
>> +	ghcb_base = (void **)this_cpu_ptr(ms_hyperv.ghcb_base);
>> +	hv_ghcb = (union hv_ghcb *)*ghcb_base;
>> +	if (!hv_ghcb) {
>> +		local_irq_restore(flags);
>> +		return;
>> +	}
>> +
>> +	memset(hv_ghcb, 0x00, HV_HYP_PAGE_SIZE);
> 
> Do you really need to zero out the whole 4k? The validation bitmap
> should be enough, there are no secrets on the page anyway.
> Same in hv_ghcb_msr_read().

OK. Thanks for suggestion. I will have a try.

> 
>> +enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
>> +				   struct es_em_ctxt *ctxt,
>> +				   u64 exit_code, u64 exit_info_1,
>> +				   u64 exit_info_2)
>>   {
>>   	enum es_result ret;
>>   
>> @@ -109,7 +109,16 @@ static enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
>>   	ghcb_set_sw_exit_info_1(ghcb, exit_info_1);
>>   	ghcb_set_sw_exit_info_2(ghcb, exit_info_2);
>>   
>> -	sev_es_wr_ghcb_msr(__pa(ghcb));
>> +	/*
>> +	 * Hyper-V runs paravisor with SEV. Ghcb page is allocated by
>> +	 * paravisor and not needs to be updated in the Linux guest.
>> +	 * Otherwise, the ghcb page's PA reported by paravisor is above
>> +	 * VTOM. Hyper-V use this function with NULL for ctxt point and
>> +	 * skip setting ghcb page in such case.
>> +	 */
>> +	if (ctxt)
>> +		sev_es_wr_ghcb_msr(__pa(ghcb));
> 
> No, do not make this function work with ctxt==NULL. Instead, factor out
> a helper function which contains what Hyper-V needs and use that in
> sev_es_ghcb_hv_call() and Hyper-V code.
> 

OK. Will update.

>> +union hv_ghcb {
>> +	struct ghcb ghcb;
>> +} __packed __aligned(PAGE_SIZE);
> 
> I am curious what this will end up being good for.
> 

Hyper-V introduces a specific hypercall request in GHCB page and use 
same union in the Linux Hyper-V code to read/write MSR and call the new 
hypercall request.