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[PATCH v2 1/6] x86/tdx: Retry TDVMCALL_MAP_GPA() when needed

Dexuan Cui posted 6 patches 2 years, 9 months ago
There is a newer version of this series
[PATCH v2 1/6] x86/tdx: Retry TDVMCALL_MAP_GPA() when needed
Posted by Dexuan Cui 2 years, 9 months ago 
GHCI spec for TDX 1.0 says that the MapGPA call may fail with the R10
error code = TDG.VP.VMCALL_RETRY (1), and the guest must retry this
operation for the pages in the region starting at the GPA specified
in R11.

When a TDX guest runs on Hyper-V, Hyper-V returns the retry error
when hyperv_init() -> swiotlb_update_mem_attributes() ->
set_memory_decrypted() decrypts up to 1GB of swiotlb bounce buffers.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
---

Changes in v2:
  Used __tdx_hypercall() directly in tdx_map_gpa().
  Added a max_retry_cnt of 1000.
  Renamed a few variables, e.g., r11 -> map_fail_paddr.

 arch/x86/coco/tdx/tdx.c | 64 +++++++++++++++++++++++++++++++++--------
 1 file changed, 52 insertions(+), 12 deletions(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 3fee96931ff5..cdeda698d308 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -20,6 +20,8 @@
 /* TDX hypercall Leaf IDs */
 #define TDVMCALL_MAP_GPA		0x10001
 
+#define TDVMCALL_STATUS_RETRY		1
+
 /* MMIO direction */
 #define EPT_READ	0
 #define EPT_WRITE	1
@@ -692,14 +694,15 @@ static bool try_accept_one(phys_addr_t *start, unsigned long len,
 }
 
 /*
- * Inform the VMM of the guest's intent for this physical page: shared with
- * the VMM or private to the guest.  The VMM is expected to change its mapping
- * of the page in response.
+ * 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>".
  */
-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);
+	int max_retry_cnt = 1000, retry_cnt = 0;
+	struct tdx_hypercall_args args;
+	u64 map_fail_paddr, ret;
 
 	if (!enc) {
 		/* Set the shared (decrypted) bits: */
@@ -707,12 +710,49 @@ 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 (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(&args, TDX_HCALL_HAS_OUTPUT);
+		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;
+
+		if (map_fail_paddr == start) {
+			retry_cnt++;
+			if (retry_cnt > max_retry_cnt)
+				return false;
+		} else {
+			retry_cnt = 0;
+			start = map_fail_paddr;
+		}
+	}
+
+	return !ret;
+}
+
+/*
+ * Inform the VMM of the guest's intent for this physical page: shared with
+ * the VMM or private to the guest. The VMM is expected to change its mapping
+ * of the page in response.
+ */
+static bool tdx_enc_status_changed(unsigned long vaddr, int numpages, bool enc)
+{
+	phys_addr_t start = __pa(vaddr);
+	phys_addr_t end   = __pa(vaddr + numpages * PAGE_SIZE);
+
+	if (!tdx_map_gpa(start, end, enc))
 		return false;
 
 	/* private->shared conversion  requires only MapGPA call */
-- 
2.25.1
Re: [PATCH v2 1/6] x86/tdx: Retry TDVMCALL_MAP_GPA() when needed
Posted by Kirill A. Shutemov 2 years, 9 months ago 
On Tue, Dec 06, 2022 at 04:33:20PM -0800, Dexuan Cui wrote:
> GHCI spec for TDX 1.0 says that the MapGPA call may fail with the R10
> error code = TDG.VP.VMCALL_RETRY (1), and the guest must retry this
> operation for the pages in the region starting at the GPA specified
> in R11.
> 
> When a TDX guest runs on Hyper-V, Hyper-V returns the retry error
> when hyperv_init() -> swiotlb_update_mem_attributes() ->
> set_memory_decrypted() decrypts up to 1GB of swiotlb bounce buffers.
> 
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> ---
> 
> Changes in v2:
>   Used __tdx_hypercall() directly in tdx_map_gpa().
>   Added a max_retry_cnt of 1000.
>   Renamed a few variables, e.g., r11 -> map_fail_paddr.
> 
>  arch/x86/coco/tdx/tdx.c | 64 +++++++++++++++++++++++++++++++++--------
>  1 file changed, 52 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 3fee96931ff5..cdeda698d308 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -20,6 +20,8 @@
>  /* TDX hypercall Leaf IDs */
>  #define TDVMCALL_MAP_GPA		0x10001
>  
> +#define TDVMCALL_STATUS_RETRY		1
> +
>  /* MMIO direction */
>  #define EPT_READ	0
>  #define EPT_WRITE	1
> @@ -692,14 +694,15 @@ static bool try_accept_one(phys_addr_t *start, unsigned long len,
>  }
>  
>  /*
> - * Inform the VMM of the guest's intent for this physical page: shared with
> - * the VMM or private to the guest.  The VMM is expected to change its mapping
> - * of the page in response.
> + * 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>".
>   */
> -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);
> +	int max_retry_cnt = 1000, retry_cnt = 0;

Hm. max_retry_cnt looks too high to me. I expected to see 3 or something.

Any justification for it to be *that* high?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov
RE: [PATCH v2 1/6] x86/tdx: Retry TDVMCALL_MAP_GPA() when needed
Posted by Dexuan Cui 2 years, 9 months ago 
> From: Kirill A. Shutemov <kirill@shutemov.name>
> Sent: Thursday, December 8, 2022 11:48 AM
> To: Dexuan Cui <decui@microsoft.com>
> > [...]
> > +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);
> > +	int max_retry_cnt = 1000, retry_cnt = 0;
> 
> Hm. max_retry_cnt looks too high to me. I expected to see 3 or something.
> 
> Any justification for it to be *that* high?

No. I just used an enough big number :-)
I'll change it to 3 in the next version.

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