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

[Dexuan Cui]

> 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?

> > +	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/