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

From: Juergen Gross
Date: Fri Dec 02 2022 - 08:40:06 EST

On 02.12.22 14:27, Kirill A. Shutemov wrote:
On Fri, Dec 02, 2022 at 06:56:47AM +0100, Juergen Gross wrote:
On 02.12.22 00:57, Kirill A. Shutemov wrote:
On Thu, Dec 01, 2022 at 05:33:28PM +0100, Juergen Gross wrote:
On 01.12.22 17:26, Kirill A. Shutemov wrote:
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

What was the solution before?

I guess MTRR was disabled, so there was no PAT, too?

Right:

Linus' tree:

[ 0.002589] last_pfn = 0x480000 max_arch_pfn = 0x10000000000
[ 0.003976] Disabled
[ 0.004452] x86/PAT: MTRRs disabled, skipping PAT initialization too.
[ 0.005856] CPU MTRRs all blank - virtualized system.
[ 0.006915] x86/PAT: Configuration [0-7]: WB WT UC- UC WB WT UC- UC

tip/master:

[ 0.003443] last_pfn = 0x20b8e max_arch_pfn = 0x10000000000
[ 0.005220] Disabled
[ 0.005818] x86/PAT: Configuration [0-7]: WB WC UC- UC WB WP UC- WT
[ 0.007752] tdx: Unexpected #VE: 28

The dangling "Disabled" comes mtrr_bp_init().


If this is the case, you can go the same route as Xen PV guests do.

Any reason X86_FEATURE_HYPERVISOR cannot be used instead of
X86_FEATURE_XENPV there?

Do we have any virtualized platform that supports it?

Yes, of course. Any hardware virtualized guest should be able to use it,
obviously TDX guests are the first ones not being able to do so.

And above dmesg snipplets are showing rather nicely that not disabling
PAT completely should be a benefit for TDX guests, as all caching modes
would be usable (the PAT MSR seems to be initialized quite fine).

Instead of X86_FEATURE_XENPV we could introduce something like
X86_FEATURE_PAT_READONLY, which could be set for Xen PV guests and for
TDX guests.

Technically, the MSR is writable on TDX. But it seems there's no way to
properly change it, following the protocol of changing on MP systems.

Why not? I don't see why it is possible in a non-TDX system, but not within
a TDX guest.

Although, I don't quite follow what role cache disabling playing on system
with self-snoop support. Hm?

It is the recommended way to do it. See SDM Vol. 3 Chapter 11 ("Memory Cache
Control"):

The operating system is responsible for insuring that changes to a PAT entry
occur in a manner that maintains the consistency of the processor caches and
translation lookaside buffers (TLB). This is accomplished by following the
procedure as specified in Section 11.11.8, “MTRR Considerations in MP Systems,
”for changing the value of an MTRR in a multiple processor system. It requires
a specific sequence of operations that includes flushing the processors caches
and TLBs.

And the sequence for the MTRRs is:

1. Broadcast to all processors to execute the following code sequence.
2. Disable interrupts.
3. Wait for all processors to reach this point.
4. Enter the no-fill cache mode. (Set the CD flag in control register CR0 to 1
and the NW flag to 0.)
5. Flush all caches using the WBINVD instructions. Note on a processor that
supports self-snooping, CPUID feature flag bit 27, this step is unnecessary.
6. If the PGE flag is set in control register CR4, flush all TLBs by clearing
that flag.
7. If the PGE flag is clear in control register CR4, flush all TLBs by executing
a MOV from control register CR3 to another register and then a MOV from that
register back to CR3.
8. Disable all range registers (by clearing the E flag in register MTRRdefType).
If only variable ranges are being modified, software may clear the valid bits
for the affected register pairs instead.
9. Update the MTRRs.
10. Enable all range registers (by setting the E flag in register MTRRdefType).
If only variable-range registers were modified and their individual valid
bits were cleared, then set the valid bits for the affected ranges instead.
11. Flush all caches and all TLBs a second time. (The TLB flush is required for
Pentium 4, Intel Xeon, and P6 family processors. Executing the WBINVD
instruction is not needed when using Pentium 4, Intel Xeon, and P6 family
processors, but it may be needed in future systems.)
12. Enter the normal cache mode to re-enable caching. (Set the CD and NW flags
in control register CR0 to 0.)
13. Set PGE flag in control register CR4, if cleared in Step 6 (above).
14. Wait for all processors to reach this point.
15. Enable interrupts.

So cache disabling is recommended.


Juergen

Attachment: OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key

Attachment: OpenPGP_signature
Description: OpenPGP digital signature