Revert "Revert Linux's PAT change and use default Xen's config again"

It looks like this workaround is still needed, at least for the nvidia
driver.

This reverts commit 0dcde71.

Fixes QubesOS/qubes-issues#9501

1018-x86-Use-Linux-s-PAT.patch

@@ -0,0 +1,81 @@
+From 7f3f94f443f652e4a59c111d4cf8b5fb3b771612 Mon Sep 17 00:00:00 2001
+From: Demi Marie Obenour <[email protected]>
+Date: Sun, 4 Dec 2022 07:57:44 -0500
+Subject: [PATCH] x86: Use Linux's PAT
+
+This is purely for testing, to see if it works around a bug in i915.  It
+is not intended to be merged.
+
+NOT-signed-off-by: DO NOT MERGE
+---
+ xen/arch/x86/include/asm/page.h      |  4 ++--
+ xen/arch/x86/include/asm/processor.h | 10 +++++-----
+ xen/arch/x86/mm.c                    |  8 --------
+ 3 files changed, 7 insertions(+), 15 deletions(-)
+
+diff --git a/xen/arch/x86/include/asm/page.h b/xen/arch/x86/include/asm/page.h
+index 350d1fb1100f..2624cc6bdae5 100644
+--- a/xen/arch/x86/include/asm/page.h
++++ b/xen/arch/x86/include/asm/page.h
+@@ -333,11 +333,11 @@ void efi_update_l4_pgtable(unsigned int l4idx, l4_pgentry_t l4e);
+
+ /* Memory types, encoded under Xen's choice of MSR_PAT. */
+ #define _PAGE_WB         (                                0)
+-#define _PAGE_WT         (                        _PAGE_PWT)
++#define _PAGE_WC         (                        _PAGE_PWT)
+ #define _PAGE_UCM        (            _PAGE_PCD            )
+ #define _PAGE_UC         (            _PAGE_PCD | _PAGE_PWT)
+-#define _PAGE_WC         (_PAGE_PAT                        )
+ #define _PAGE_WP         (_PAGE_PAT |             _PAGE_PWT)
++#define _PAGE_WT         (_PAGE_PAT | _PAGE_PCD | _PAGE_PWT)
+
+ /*
+  * Debug option: Ensure that granted mappings are not implicitly unmapped.
+diff --git a/xen/arch/x86/include/asm/processor.h b/xen/arch/x86/include/asm/processor.h
+index c26ef9090c3a..ec881b880d4d 100644
+--- a/xen/arch/x86/include/asm/processor.h
++++ b/xen/arch/x86/include/asm/processor.h
+@@ -65,16 +65,16 @@
+
+ /*
+  * Host IA32_CR_PAT value to cover all memory types.  This is not the default
+- * MSR_PAT value, and is an ABI with PV guests.
++ * MSR_PAT value, and is needed by the Linux i915 driver.
+  */
+ #define XEN_MSR_PAT ((_AC(X86_MT_WB,  ULL) << 0x00) | \
+-                     (_AC(X86_MT_WT,  ULL) << 0x08) | \
++                     (_AC(X86_MT_WC,  ULL) << 0x08) | \
+                      (_AC(X86_MT_UCM, ULL) << 0x10) | \
+                      (_AC(X86_MT_UC,  ULL) << 0x18) | \
+-                     (_AC(X86_MT_WC,  ULL) << 0x20) | \
++                     (_AC(X86_MT_WB,  ULL) << 0x20) | \
+                      (_AC(X86_MT_WP,  ULL) << 0x28) | \
+-                     (_AC(X86_MT_UC,  ULL) << 0x30) | \
+-                     (_AC(X86_MT_UC,  ULL) << 0x38))
++                     (_AC(X86_MT_UCM, ULL) << 0x30) | \
++                     (_AC(X86_MT_WT,  ULL) << 0x38))
+
+ #ifndef __ASSEMBLY__
+
+diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
+index 702c76a879e5..b30453b9de11 100644
+--- a/xen/arch/x86/mm.c
++++ b/xen/arch/x86/mm.c
+@@ -6365,14 +6365,6 @@ unsigned long get_upper_mfn_bound(void)
+  */
+ static void __init __maybe_unused build_assertions(void)
+ {
+-    /*
+-     * If this trips, any guests that blindly rely on the public API in xen.h
+-     * (instead of reading the PAT from Xen, as Linux 3.19+ does) will be
+-     * broken.  Furthermore, live migration of PV guests between Xen versions
+-     * using different PATs will not work.
+-     */
+-    BUILD_BUG_ON(XEN_MSR_PAT != 0x050100070406ULL);
+-
+     /*
+      * _PAGE_WB must be zero for several reasons, not least because Linux
+      * assumes it.
+-- 
+2.44.0
+

xen.spec.in

@@ -162,6 +162,7 @@ Patch1014: 1014-libxl-conditionally-allow-PCI-passthrough-on-PV-with.patch
 Patch1015: 1015-gnttab-disable-grant-tables-v2-by-default.patch
 Patch1016: 1016-cpufreq-enable-HWP-by-default.patch
 Patch1017: 1017-Fix-IGD-passthrough-with-linux-stubdomain.patch
+Patch1018: 1018-x86-Use-Linux-s-PAT.patch
 
 # Reproducible builds
 Patch1100: 1100-Define-build-dates-time-based-on-SOURCE_DATE_EPOCH.patch