Re: [edk2-devel] [PATCH V1 1/1] UefiCpuPkg/ResetVector: Cache Disable should not be set by default in CR0

["Gerd Hoffmann" <kraxel@redhat.com>]

On Mon, Jan 22, 2024 at 01:11:52PM -0600, Brian J. Johnson wrote:
> On 1/18/24 09:46, Gerd Hoffmann wrote:
> > On Wed, Jan 10, 2024 at 04:43:47PM +0000, West, Catharine wrote:
> > > Disabling cache by default results in violation of BTG protections (if BTG enabled).
> > > BIOS cannot assume that cache is disabled before it executes as ACM may be required to enable NEM.
> > > 
> > > Whatever solution needs to be done here cannot evict ACM-enabled NEM.
> > 
> > Well, it's OVMF in a virtual machine.  No boot guard involved.
> > So we could probably go for a OVMF-specific patch here.
> > 
> > But I'd prefer to figure what exactly is happening here before going
> > down that route.  An extreme slowdown just because we flip that bit
> > doesn't make sense to me.
> > 
> > > Why is boot time increasing?
> > 
> > Not clear.  It seems to be the lzma uncompress of the firmware volume
> > in rom / pflash which is very slow.  Also it is apparently only
> > triggered in case pci device assignment is used.
> 
> I've seen extreme slowness on physical platforms when we've mixed up the
> MTRRs or page tables, causing code to be mapped uncached.
> 
> Lzma uncompress of ROM could be pretty slow as well, if the ROM is being
> read uncached.  Lzma probably reads the data a byte at a time, which is the
> worst case for uncached accesses.  Since this is a VM, it's not actually
> uncached at the hardware level, but I don't know how QEMU/KVM handles
> uncached guest mappings.... It may be doing a VMEXIT for every byte.
> 
> Anyway, I suggest double-checking your page tables and MTRRs.

It happens very early at boot, before MTRRs are setup, running on the
initial page tables created by the OVMF reset vector.  The initial page
tables have just 'accessed', 'dirty', 'read/write' and 'present' bits
set for the 0-4G identity mapping.

It seems to have something to do with EPT.  It does not happen on AMD
processors.  It also does not happen when disabling EPT support in kvm
on the host machine.

looked at kvm kernel traces, I don't see excessive vmexits.

take care,
  Gerd



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