Re: [edk2-devel] [PATCH V2 1/4] OvmfPkg/IoMmuDxe: Reserve shared memory region for DMA operation

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

On December 14, 2022 12:04 AM, Lendacky, Thomas wrote:
> 
> On 12/12/22 23:48, Min Xu wrote:
> > From: Min M Xu <min.m.xu@intel.com>
> > BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4171
> >
> 
> This patch causes crashes for SEV guests and breaks bisect-ability of the
> EDK2 tree. See below...
> 
> >   /**
> > @@ -878,6 +860,11 @@ IoMmuUnmapAllMappings (
> >         TRUE      // MemoryMapLocked
> >         );
> >     }
> > +
> > +  //
> > +  // Release the reserved shared memory as well.
> > +  //
> > +  IoMmuReleaseReservedSharedMem (TRUE);
> 
> This call is the reason for the crash. You'll need to check for whether there
> has been any shared memory reserved before attempting to free it (in the
> case of SEV that doesn't happen until patch #3 of the series). I think adding
> the check in IoMmuReleaseReservedSharedMem() itself might be best, since
> you can also experience this crash should the below allocation fail, too.
> 
Ah, yes. In IoMmuReleaseReservedSharedMem we should first check mReservedSharedMemSupported to see if there is any reserved memory. I test it in my side and there is no crash. 
Tom, if this check is added, do you experience any crash?

Thanks
Min