Re: [edk2-devel] Regression: 100x I/O performance slowdown in SEC phase caused by TDX

["Min Xu" <min.m.xu@intel.com>]

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