On April 13, 2022 5:22 AM, James Bottom wrote:
>
> I'm using a SEC phase which has a TPM driver to experiment with sorting out
> measured boot, which is how I noticed (usually SEC doesn't do MMIO) .
> What I'm seeing is after commit b6b2de884864 ("MdePkg: Support mmio for
> Tdx guest in BaseIoLibIntrinsic") we get a massive slowdown of about 100x in
> TPM performance. The reason seems to be this addition to the
> mmioreadX/mmiowriteX code:
>
> MemoryFence ();
> - *(volatile UINT16 *)Address = Value;
> +
> + if (IsTdxGuest ()) {
> + TdMmioWrite16 (Address, Value);
> + } else {
> + *(volatile UINT16 *)Address = Value;
> + }
> +
> MemoryFence ();
>
>
> The problem is that IsTdxGuest () has this structure:
>
> BOOLEAN
> EFIAPI
> IsTdxGuest (
> VOID
> )
> {
> if (mTdxProbed) {
> return mTdxEnabled;
> }
>
> mTdxEnabled = TdIsEnabled ();
> mTdxProbed = TRUE;
>
> return mTdxEnabled;
> }
>
> Which 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.
>
The analysis is right. I hadn't thought the basic IO would be heavily called in
SEC phase. I will fix it as soon as possible.
My thought is that a TdProbeLib will be introduced in MdePkg. In MdePkg this
lib is of null instance. It just simply return TD_PROBE_NON which means it is
not Td guest. This lib will be added in MdePkg/MdeLibs.dsc.inc (like
RegisterFilterLibNull).
In OvmfPkg TdProbeLib will be implemented. It checks the OvmfWorkArea to
determine the guest type.
Thanks
Min
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