Re: [PATCH V2 5/6] MdePkg: Probe Cc guest in BaseIoLibIntrinsicSev

dann frazier

On Fri, Apr 15, 2022 at 08:07:08AM +0800, Min Xu wrote:

Bad IO performance in SEC phase is observed after TDX features was
introduced. (after commit b6b2de884864 - "MdePkg: Support mmio for
Tdx guest in BaseIoLibIntrinsic").

This is because IsTdxGuest() will be called in each MMIO operation.
It is trying to cache the result of the probe in the efi data segment.
However, that doesn't work in SEC, because the data segment is read only
(so the write seems to succeed but a read will always return the
original value), leading to us calling TdIsEnabled() check for every
mmio we do, which is causing the slowdown because it's very expensive.

This patch is to call CcProbe instead of TdIsEnabled in IsTdxGuest.
Null instance of CcProbe always returns CCGuestTypeNonEncrypted. Its
OvmfPkg version returns the guest type in Ovmf work area.

I ran through our tests on stable-202205-rc1, and I'm finding that all
of the tests using 2M FD_SIZE & SMM_REQUIRE=TRUE are failing with
QEMU hanging w/o output. Equivalent tests w/ 4M FD_SIZE are working
fine. I bisected it down to this commit, and also confirmed that
reverting this commit on top of 202205-rc1 also avoids the problem.

I might have a chance to debug more tomorrow, but for now I just
wanted to flag it.


Cc: Michael D Kinney <michael.d.kinney@...>
Cc: Liming Gao <gaoliming@...>
Cc: Zhiguang Liu <zhiguang.liu@...>
Cc: James Bottomley <jejb@...>
Cc: James Bottomley <James.Bottomley@...>
Cc: Jiewen Yao <jiewen.yao@...>
Cc: Gerd Hoffmann <kraxel@...>
Signed-off-by: Min Xu <min.m.xu@...>
.../BaseIoLibIntrinsic/BaseIoLibIntrinsicSev.inf | 1 +
.../Library/BaseIoLibIntrinsic/IoLibInternalTdx.c | 13 ++-----------
2 files changed, 3 insertions(+), 11 deletions(-)

diff --git a/MdePkg/Library/BaseIoLibIntrinsic/BaseIoLibIntrinsicSev.inf b/MdePkg/Library/BaseIoLibIntrinsic/BaseIoLibIntrinsicSev.inf
index 7fe1c60f046e..e1b8298ac451 100644
--- a/MdePkg/Library/BaseIoLibIntrinsic/BaseIoLibIntrinsicSev.inf
+++ b/MdePkg/Library/BaseIoLibIntrinsic/BaseIoLibIntrinsicSev.inf
@@ -55,6 +55,7 @@
+ CcProbeLib

diff --git a/MdePkg/Library/BaseIoLibIntrinsic/IoLibInternalTdx.c b/MdePkg/Library/BaseIoLibIntrinsic/IoLibInternalTdx.c
index 1e539dbfbbad..8af6fc35c591 100644
--- a/MdePkg/Library/BaseIoLibIntrinsic/IoLibInternalTdx.c
+++ b/MdePkg/Library/BaseIoLibIntrinsic/IoLibInternalTdx.c
@@ -10,6 +10,7 @@
#include <Include/IndustryStandard/Tdx.h>
#include <Library/TdxLib.h>
#include <Register/Intel/Cpuid.h>
+#include <Library/CcProbeLib.h>
#include "IoLibTdx.h"

// Size of TDVMCALL Access, including IO and MMIO
@@ -22,9 +23,6 @@

-BOOLEAN mTdxEnabled = FALSE;
-BOOLEAN mTdxProbed = FALSE;
Check if it is Tdx guest.

@@ -38,14 +36,7 @@ IsTdxGuest (
- if (mTdxProbed) {
- return mTdxEnabled;
- }
- mTdxEnabled = TdIsEnabled ();
- mTdxProbed = TRUE;
- return mTdxEnabled;
+ return CcProbe () == CCGuestTypeIntelTdx;


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