Re: [PATCH 0/4] Introduce TdProbe in MdePkg

["James Bottomley" <jejb@linux.ibm.com>]

On Wed, 2022-04-13 at 17:08 +0800, Min Xu wrote:
> BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3902
> 
> 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.
> 
> TdProbe is introduced in this patch-set. It is called in
> BaseIoLibIntrinsicSev instead of IsTdxGuest. There are 2 versions of
> the TdProbeLib. Null instance of TdProbe always returns TD_PROBE_NON.
> Its OvmfPkg version checks the Ovmf work area to determine the Td
> guest type.

I tested this out with the TPM code: it restores pretty much all of the
lost performance, thanks!

Tested-by: James Bottomley <jejb@linux.ibm.com>

James