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

Dexuan Cui
2023-05-25T02:06:31Z

> From: Dave Hansen <dave.hansen@intel.com>
> 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;
+}