Re: [PATCH v5 13/16] x86: decouple PAT and MTRR handling

From: Kirill A. Shutemov
Date: Thu Dec 01 2022 - 11:26:50 EST


On Wed, Nov 02, 2022 at 08:47:10AM +0100, Juergen Gross wrote:
> Today PAT is usable only with MTRR being active, with some nasty tweaks
> to make PAT usable when running as Xen PV guest, which doesn't support
> MTRR.
>
> The reason for this coupling is, that both, PAT MSR changes and MTRR
> changes, require a similar sequence and so full PAT support was added
> using the already available MTRR handling.
>
> Xen PV PAT handling can work without MTRR, as it just needs to consume
> the PAT MSR setting done by the hypervisor without the ability and need
> to change it. This in turn has resulted in a convoluted initialization
> sequence and wrong decisions regarding cache mode availability due to
> misguiding PAT availability flags.
>
> Fix all of that by allowing to use PAT without MTRR and by reworking
> the current PAT initialization sequence to match better with the newly
> introduced generic cache initialization.
>
> This removes the need of the recently added pat_force_disabled flag, so
> remove the remnants of the patch adding it.
>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

This patch breaks boot for TDX guest.

Kernel now tries to set CR0.CD which is forbidden in TDX guest[1] and
causes #VE:

tdx: Unexpected #VE: 28
VE fault: 0000 [#1] PREEMPT SMP NOPTI
CPU: 0 PID: 0 Comm: swapper Not tainted 6.1.0-rc1-00015-gadfe7512e1d0 #2646
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
RIP: 0010:native_write_cr0 (arch/x86/kernel/cpu/common.c:427)
Call Trace:
<TASK>
? cache_disable (arch/x86/include/asm/cpufeature.h:173 arch/x86/kernel/cpu/cacheinfo.c:1085)
? cache_cpu_init (arch/x86/kernel/cpu/cacheinfo.c:1132 (discriminator 3))
? setup_arch (arch/x86/kernel/setup.c:1079)
? start_kernel (init/main.c:279 (discriminator 3) init/main.c:477 (discriminator 3) init/main.c:960 (discriminator 3))
? load_ucode_bsp (arch/x86/kernel/cpu/microcode/core.c:155)
? secondary_startup_64_no_verify (arch/x86/kernel/head_64.S:358)
</TASK>

Any suggestion how to fix it?

[1] Section 10.6.1. "CR0", https://cdrdv2.intel.com/v1/dl/getContent/733568
--
Kiryl Shutsemau / Kirill A. Shutemov