[PATCH v6 0/5] Make PAT handling less brittle

From: Demi Marie Obenour <demi@invisiblethingslab.com>
To: xen-devel@lists.xenproject.org
Cc: "Demi Marie Obenour" <demi@invisiblethingslab.com>,
	"Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>,
	"Jan Beulich" <jbeulich@suse.com>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>, "Wei Liu" <wl@xen.org>,
	"Tim Deegan" <tim@xen.org>,
	"George Dunlap" <george.dunlap@citrix.com>
Subject: [PATCH v6 0/5] Make PAT handling less brittle
Date: Thu, 22 Dec 2022 17:31:45 -0500	[thread overview]
Message-ID: <cover.1671744225.git.demi@invisiblethingslab.com> (raw)

While working on Qubes OS Marek found out that there were some PAT hacks
in the Linux i195 driver.  I decided to make Xen use Linux’s PAT to see
if it solved the graphics glitches that were observed; it did.  This
required a substantial amount of preliminary work that is useful even
without using Linux’s PAT.

Patches 1 through 4 are the preliminary work and I would like them to be
accepted into upstream Xen.  Patch 4 does break ABI by rejecting the
unused PAT entries, but this will only impact buggy PV guests and can be
disabled with a Xen command-line option.  Patch 5 actually switches to
Linux’s PAT and is NOT intended to be merged (at least for now) as it
would at a minimum break migration of PV guests from hosts that do not
have the patch.

Only patches 4 and 5 actually change Xen’s observable behavior.
Patch 1 is a prerequisite that removes the last place where Xen
hard-codes its current PAT value.  Patch 2 is strictly cleanup.  Patch 3
makes changing the PAT much less error-prone, as problems with the PAT
or with the associated _PAGE_* constants will be detected at compile
time.

Demi Marie Obenour (5):
  x86/mm: Avoid hard-coding PAT in get_page_from_l1e()
  x86: Remove MEMORY_NUM_TYPES and NO_HARDCODE_MEM_TYPE
  x86/mm: make code robust to future PAT changes
  x86/mm: Reject invalid cacheability in PV guests by default
  x86: Use Linux's PAT

 docs/misc/xen-command-line.pandoc    |  11 ++
 xen/arch/x86/hvm/mtrr.c              |  18 ++--
 xen/arch/x86/include/asm/mtrr.h      |   2 -
 xen/arch/x86/include/asm/page.h      |   4 +-
 xen/arch/x86/include/asm/processor.h |  10 +-
 xen/arch/x86/mm.c                    | 146 ++++++++++++++++++++++++---
 xen/arch/x86/mm/shadow/multi.c       |   2 +-
 7 files changed, 162 insertions(+), 31 deletions(-)

-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab