[PATCH v6 4/6] virt: tdx-guest: Add a helper for the Quote buffer size

[Peter Fang <peter.fang@intel.com>]

The Quote buffer size is a constant sprinkled across several places.
Read it from a helper instead. And rename the constant to avoid using a
verb.

This sets up the plumbing for a later change that makes the size
dynamic.

There are several "sizes" at play here: the raw Quote data, the Quote
buffer (the header metadata plus the raw data), and the allocation (the
page-padded Quote buffer). The helper returns the Quote buffer size and
leaves page alignment to callers that need it.

AI was used under supervision to collect/apply feedback, review code and
workshop logs.

Signed-off-by: Peter Fang <peter.fang@intel.com>
---
v6:
 - Don't cache the size in the helper. [Rick]
 - Let the callers page-align the buffer size. [Rick]
 - Add a comment for the helper. [Dave]
v5:
 - Reworked v4 3/4 to use a helper instead of a global. [Dave, Xiaoyao]
 - Use TDX_DEFAULT_QUOTE_SIZE instead of GET_QUOTE_DEFAULT_BUF_SIZE.
   [Dave]
 - Use "size" instead of "len" for buffer size variables. [Dave]
 - Drop the RB tags, as the code changed substantially.
---
 drivers/virt/coco/tdx-guest/tdx-guest.c | 31 +++++++++++++++++--------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/drivers/virt/coco/tdx-guest/tdx-guest.c b/drivers/virt/coco/tdx-guest/tdx-guest.c
index 77c63c3f820f..0cf078f09a73 100644
--- a/drivers/virt/coco/tdx-guest/tdx-guest.c
+++ b/drivers/virt/coco/tdx-guest/tdx-guest.c
@@ -162,7 +162,7 @@ static void tdx_mr_deinit(const struct attribute_group *mr_grp)
  * DICE-based attestation uses layered evidence that requires
  * larger Quote size (~100K).
  */
-#define GET_QUOTE_BUF_SIZE		SZ_128K
+#define TDX_DEFAULT_QUOTE_SIZE		SZ_128K
 
 #define GET_QUOTE_CMD_VER		1
 
@@ -206,26 +206,36 @@ static long tdx_get_report0(struct tdx_report_req __user *req)
 			     USER_SOCKPTR(req->tdreport));
 }
 
+/* Size of the header metadata plus the largest possible raw Quote. */
+static size_t get_quote_buf_size(void)
+{
+	return TDX_DEFAULT_QUOTE_SIZE;
+}
+
 static void free_quote_buf(struct tdx_quote_buf *buf)
 {
-	size_t len = PAGE_ALIGN(GET_QUOTE_BUF_SIZE);
-	unsigned int count = len >> PAGE_SHIFT;
+	size_t alloc_size = PAGE_ALIGN(get_quote_buf_size());
+	unsigned int count;
+
+	count = alloc_size >> PAGE_SHIFT;
 
 	if (set_memory_encrypted((unsigned long)buf, count)) {
 		pr_err("Failed to restore encryption mask for Quote buffer, leak it\n");
 		return;
 	}
 
-	free_pages_exact(buf, len);
+	free_pages_exact(buf, alloc_size);
 }
 
 static struct tdx_quote_buf *alloc_quote_buf(void)
 {
-	size_t len = PAGE_ALIGN(GET_QUOTE_BUF_SIZE);
-	unsigned int count = len >> PAGE_SHIFT;
+	size_t alloc_size = PAGE_ALIGN(get_quote_buf_size());
 	struct tdx_quote_buf *buf;
+	unsigned int count;
 
-	buf = alloc_pages_exact(len, GFP_KERNEL | __GFP_ZERO);
+	count = alloc_size >> PAGE_SHIFT;
+
+	buf = alloc_pages_exact(alloc_size, GFP_KERNEL | __GFP_ZERO);
 	if (!buf)
 		return NULL;
 
@@ -266,6 +276,7 @@ static int tdx_report_new_locked(struct tsm_report *report)
 {
 	u8 *buf;
 	struct tsm_report_desc *desc = &report->desc;
+	size_t quote_buf_size = get_quote_buf_size();
 	u32 out_len;
 	int ret;
 	u64 err;
@@ -281,7 +292,7 @@ static int tdx_report_new_locked(struct tsm_report *report)
 	if (desc->inblob_len != TDX_REPORTDATA_LEN)
 		return -EINVAL;
 
-	memset(quote_buf, 0, GET_QUOTE_BUF_SIZE);
+	memset(quote_buf, 0, quote_buf_size);
 
 	/* Update Quote buffer header */
 	quote_buf->version = GET_QUOTE_CMD_VER;
@@ -292,7 +303,7 @@ static int tdx_report_new_locked(struct tsm_report *report)
 	if (ret)
 		return ret;
 
-	err = tdx_hcall_get_quote(quote_buf, GET_QUOTE_BUF_SIZE);
+	err = tdx_hcall_get_quote(quote_buf, PAGE_ALIGN(quote_buf_size));
 	if (err) {
 		pr_err("GetQuote hypercall failed, status:%llx\n", err);
 		return -EIO;
@@ -311,7 +322,7 @@ static int tdx_report_new_locked(struct tsm_report *report)
 
 	out_len = READ_ONCE(quote_buf->out_len);
 
-	if (struct_size(quote_buf, data, out_len) > GET_QUOTE_BUF_SIZE)
+	if (struct_size(quote_buf, data, out_len) > quote_buf_size)
 		return -EFBIG;
 
 	buf = kvmemdup(quote_buf->data, out_len, GFP_KERNEL);
-- 
2.53.0