On 8/17/2023 6:01 AM, Sean Christopherson wrote:
On Wed, Jul 19, 2023, Binbin Wu wrote:
+ return (sign_extend64(gva, lam_bit) & ~BIT_ULL(63)) | (gva & BIT_ULL(63));
Almost forgot. Please add a comment explaning how LAM untags the address,
specifically the whole bit 63 preservation. The logic is actually straightforward,
but the above looks way more complex than it actually is. This?
/*
* Untag the address by sign-extending the LAM bit, but NOT to bit 63.
* Bit 63 is retained from the raw virtual address so that untagging
* doesn't change a user access to a supervisor access, and vice versa.
*/
OK.
Besides it, I find I forgot adding the comments for the function. I will
add it back if you don't object.
+/*
+ * Only called in 64-bit mode.
+ *
+ * LAM has a modified canonical check when applicable:
+ * LAM_S48 : [ 1 ][ metadata ][ 1 ]
+ * 63 47
+ * LAM_U48 : [ 0 ][ metadata ][ 0 ]
+ * 63 47
+ * LAM_S57 : [ 1 ][ metadata ][ 1 ]
+ * 63 56
+ * LAM_U57 + 5-lvl paging : [ 0 ][ metadata ][ 0 ]
+ * 63 56
+ * LAM_U57 + 4-lvl paging : [ 0 ][ metadata ][ 0...0 ]
+ * 63 56..47
+ *
+ * Note that KVM masks the metadata in addresses, performs the (original)
+ * canonicality checking and then walks page table. This is slightly
+ * different from hardware behavior but achieves the same effect.
+ * Specifically, if LAM is enabled, the processor performs a modified
+ * canonicality checking where the metadata are ignored instead of
+ * masked. After the modified canonicality checking, the processor masks
+ * the metadata before passing addresses for paging translation.
+ */
