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

["Laszlo Ersek" <lersek@redhat.com>]

On 1/23/24 11:52, Gerd Hoffmann wrote:
> 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.

This discussion evokes vague memories in me. I'll dump them here, but I
have no idea if they will be useful. (They probably won't.)

- edk2 commit 98f378a7be12 ("OvmfPkg/ResetVector: enable caching in
initial page tables", 2013-09-24)

- Linux (host) commit 879ae1880449 ("KVM: x86: obey
KVM_X86_QUIRK_CD_NW_CLEARED in kvm_set_cr0()", 2015-11-04)

Laszlo



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