[edk2-devel] [PATCH v2 09/23] OvmfPkg/BaseMemEncryptSevLib: Maximize Page State Change efficiency

["Lendacky, Thomas via groups.io" <thomas.lendacky=amd.com@groups.io>]

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4654

When building the Page State Change entries for a range of memory, it can
happen that multiple calls to BuildPageStateBuffer() need to be made. If
the size of the input work area passed to BuildPageStateBuffer() exceeds
the number of entries that can be passed to the hypervisor using the GHCB
shared buffer, the Page State Change VMGEXIT support will issue multiple
VMGEXITs to process all entries in the buffer.

However, it could be that the final VMGEXIT for each round of Page State
Changes is only for a small number of entries and subsequent VMGEXITs may
still be issued to handle the full range of memory requested. To maximize
the number of entries processed during the Page State Change VMGEXIT,
limit BuildPageStateBuffer() to not build entries that exceed the maximum
number of entries that can be handled in a single Page State Change
VMGEXIT.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 OvmfPkg/Library/BaseMemEncryptSevLib/X64/SnpPageStateChangeInternal.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/OvmfPkg/Library/BaseMemEncryptSevLib/X64/SnpPageStateChangeInternal.c b/OvmfPkg/Library/BaseMemEncryptSevLib/X64/SnpPageStateChangeInternal.c
index bcc0798d6b02..f1883239a661 100644
--- a/OvmfPkg/Library/BaseMemEncryptSevLib/X64/SnpPageStateChangeInternal.c
+++ b/OvmfPkg/Library/BaseMemEncryptSevLib/X64/SnpPageStateChangeInternal.c
@@ -145,6 +145,7 @@ BuildPageStateBuffer (
   UINTN                 RmpPageSize;
   UINTN                 Index;
   UINTN                 IndexMax;
+  UINTN                 PscIndexMax;
 
   // Clear the page state structure
   SetMem (Info, InfoSize, 0);
@@ -153,6 +154,16 @@ BuildPageStateBuffer (
   IndexMax    = (InfoSize - sizeof (Info->Header)) / sizeof (Info->Entry[0]);
   NextAddress = EndAddress;
 
+  //
+  // Make the use of the work area as efficient as possible relative to
+  // exiting from the guest to the hypervisor. Maximize the number of entries
+  // that can be processed per exit.
+  //
+  PscIndexMax = (IndexMax / SNP_PAGE_STATE_MAX_ENTRY) * SNP_PAGE_STATE_MAX_ENTRY;
+  if (PscIndexMax > 0) {
+    IndexMax = MIN (IndexMax, PscIndexMax);
+  }
+
   //
   // Populate the page state entry structure
   //
-- 
2.42.0



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