Re: [PATCH v8 1/2] x86/tdx: Retry TDVMCALL_MAP_GPA() when needed

[Sathyanarayanan Kuppuswamy]

Hi,

On 6/20/23 12:23 PM, Dexuan Cui wrote:
>> From: Sathyanarayanan Kuppuswamy
>> Sent: Tuesday, June 20, 2023 11:31 AM
>>> ...
>>> -static bool tdx_enc_status_changed(unsigned long vaddr, int numpages,
>> bool enc)
>>> +static bool tdx_map_gpa(phys_addr_t start, phys_addr_t end, bool enc)
>>>  {
>>> -	phys_addr_t start = __pa(vaddr);
>>> -	phys_addr_t end   = __pa(vaddr + numpages * PAGE_SIZE);
>>> +	const int max_retries_per_page = 3;
>>
>> Add some details about why you chose 3? Maybe you can also use macro for it.
> 
> It's a small number recommended by Kirill:
> https://lwn.net/ml/linux-kernel/20221208194800.n27ak4xj6pmyny46@xxxxxxxxxxxxxxxxx/
> 
> The spec doesn't define a max retry count. Normally I guess a max retry count
> of 2 should be enough, at least for Hyper-V according to my testing.
> 
> Maybe we can add a comment like this:
> 
> /* Retrying the hypercall a second time should succeed; use 3 just in case. */
> 
> Does this look good to all?

Looks fine to me.

> 
>>> +	struct tdx_hypercall_args args;
>>> +	u64 map_fail_paddr, ret;
>>> +	int retry_count = 0;
>>>
>>>  	if (!enc) {
>>>  		/* Set the shared (decrypted) bits: */
>>> @@ -718,12 +720,49 @@ static bool tdx_enc_status_changed(unsigned long
>> vaddr, int numpages, bool enc)
>>>  		end   |= cc_mkdec(0);
>>>  	}
>>>
>>> -	/*
>>> -	 * Notify the VMM about page mapping conversion. More info about ABI
>>> -	 * can be found in TDX Guest-Host-Communication Interface (GHCI),
>>> -	 * section "TDG.VP.VMCALL<MapGPA>"
>>> -	 */
>>> -	if (_tdx_hypercall(TDVMCALL_MAP_GPA, start, end - start, 0, 0))
>>> +	while (retry_count < max_retries_per_page) {
>>> +		memset(&args, 0, sizeof(args));
>>> +		args.r10 = TDX_HYPERCALL_STANDARD;
>>> +		args.r11 = TDVMCALL_MAP_GPA;
>>> +		args.r12 = start;
>>> +		args.r13 = end - start;
>>> +
>>> +		ret = __tdx_hypercall_ret(&args);
>>> +		if (ret != TDVMCALL_STATUS_RETRY)
>>> +			return !ret;
>>> +		/*
>>> +		 * The guest must retry the operation for the pages in the
>>> +		 * region starting at the GPA specified in R11. R11 comes
>>> +		 * from the untrusted VMM. Sanity check it.
>>> +		 */
>>> +		map_fail_paddr = args.r11;
>>
>> Do you really need map_fail_paddr? Why not directly use args.r11?
>>
>>> +		if (map_fail_paddr < start || map_fail_paddr >= end)
>>> +			return false;
> 
> Originally, I used r11. 
> 
> Dave says " 'r11' needs a real, logical name":
> https://lwn.net/ml/linux-kernel/6bb65614-d420-49d3-312f-316dc8ca4cc4@xxxxxxxxx/

Got it.

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer