RE: [PATCH v6 1/6] x86/tdx: Retry TDVMCALL_MAP_GPA() when needed

[Dexuan Cui]

> From: Dave Hansen <dave.hansen@xxxxxxxxx>
> Sent: Tuesday, May 23, 2023 2:13 PM
> ...
> On 5/4/23 15:53, Dexuan Cui wrote:
> > -	if (_tdx_hypercall(TDVMCALL_MAP_GPA, start, end - start, 0, 0))
> > +	while (1) {
> > +		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)
> > +			break;
> > +		/*
> > +		 * The guest must retry the operation for the pages in the
> > +		 * region starting at the GPA specified in R11. Make sure R11
> > +		 * contains a sane value.
> > +		 */
> > +		map_fail_paddr = args.r11;
> > +		if (map_fail_paddr < start || map_fail_paddr >= end)
> > +			return false;
> 
> This should probably also say: "r11" comes from the untrusted VMM.
> Sanity check it.

Thanks! I'll use the wording you recommended in the next version.
 
> Should this *really* be "map_fail_paddr >= end"?  Or is
> "map_fail_paddr > end" sufficient.  In other words, is it really
> worth failing this if a VMM said to retry a 0-byte region at the end?

According to the GHCI spec, R13 "must be a multiple of 4KB". My
understanding is that R13 should not be 0, and a hypervisor is not
supposed to tell the guest to retry a 0-byte region at the end.

IMHO it should be a hypervisor bug if a hypervisor returns
TDVMCALL_STATUS_RETRY and the returned 'map_fail_paddr' equals
'end' (Note: the valid page range is [start, end - 1]).

Hyper-V returns "invalid parameter" if the length (i.e. args.r13) is 0,
so "retry a 0-byte region at the end" would fail anyway. I guess
other hypervisors may also return an error if the length is 0.

So I'd like to keep the comparison as-is.

> > +		if (map_fail_paddr == start) {
> > +			retry_cnt++;
> > +			if (retry_cnt > max_retry_cnt)
> 
> I think we can spare two bytes in a few spots to make these 'count'
> instead of 'cnt'.

Ok, I'll rename the variable 'max_retry_cnt' to 'max_retries_per_page',
and 'retry_cnt' to 'retry_count'.

> > +				return false;
> > +		} else {
> > +			retry_cnt = 0;
> > +			start = map_fail_paddr;
> > +		}
> > +	}
> 
> this fails the "normal operation should be at the lowest indentation"
> rule.  How about this:
> 
> 	while (retry_count < max_retries) {
> 		...
> 
> 		/* "Consume" a retry without forward progress: */
> 		if (map_fail_paddr == start) {
> 			retry_count++;
> 			continue;
> 		}
> 
> 		start = map_fail_paddr;
> 		retry_count = 0;
> 	}
> 
> 	// plus maybe a wee bit different 'ret' processing
> 
> 
> 'max_retries' also ends up being a misnomer.  You can have as many
> retries as there are pages plus 'max_retries'.  It's really "maximum
> allowed consecutive failures".  Maybe it should be "max_retries_per_page".

Thanks, I'll raname 'max_retries" to 'max_retries_per_page'.

I'll use the beow in the next version.
I added "const" to "int max_retries_per_page".
Please let me know if I missed anything.

+static bool tdx_map_gpa(phys_addr_t start, phys_addr_t end, bool enc)
+{
+	const int max_retries_per_page = 3;
+	struct tdx_hypercall_args args;
+	u64 map_fail_paddr, ret;
+	int retry_count = 0;
+
+	if (!enc) {
+		/* Set the shared (decrypted) bits: */
+		start |= cc_mkdec(0);
+		end   |= cc_mkdec(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;
+		if (map_fail_paddr < start || map_fail_paddr >= end)
+			return false;
+
+		/* "Consume" a retry without forward progress */
+		if (map_fail_paddr == start) {
+			retry_count++;
+			continue;
+		}
+
+		start = map_fail_paddr;
+		retry_count = 0;
+	}
+
+	return false;
+}