29d3fdc7f6721c08e9c2314764d849ae425a984d4cba9358327b49c5.patch

From 9ffdcfd7ae75731328018465f61d58fcd90fa749 Mon Sep 17 00:00:00 2001
From: Yan Zhao <yan.y.zhao@intel.com>
Date: Fri, 5 Jan 2024 17:15:35 +0800
Subject: [PATCH] BACKPORT: FROMLIST: KVM: VMX: Honor guest PATs for memslots
 of flag KVM_MEM_NON_COHERENT_DMA

Honor guest PATs in the range of memslots of flag KVM_MEM_NON_COHERENT_DMA
set no matter the value of noncoherent dma count.

Just honoring guest PATs (without honoring guest MTRRs) for memslots of
flag KVM_MEM_NON_COHERENT_DMA is because
- guest OS will ensure no page aliasing issue in guest side by honoring
  guest MTRRs in guest page table.
  Combinations like guest MTRR=WC or UC, guest PAT = WB is not allowed.
  (at least in Linux, see pat_x_mtrr_type()).
- guest device driver programs device hardware according to guest PATs in
  modern platforms.

Besides, we don't break down an EPT huge page if guest MTRRs in its range
are not consistent, because
- guest should have chosen correct guest PATs according to guest MTRRs.
- in normal platforms, small guest pages with different PATs must
  correspond to different TLBs though they are mapped in a huge page in
  EPT.

However, one condition may not be supported well by honoring guest PAT
alone -- when guest MTRR=WC, guest PAT=UC-.
By honoring guest MTRRs+PATs, the effective memory type is WC; while
by honoring guest PATs alone, the effective memory type is UC.
But it's arguable to support such a usage.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Tested-by: Yongwei Ma <yongwei.ma@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Dawn Han <dawnhan@google.com>

(am from https://lore.kernel.org/all/20240105091535.24760-1-yan.y.zhao@intel.com/)

BUG=b:335525888
UPSTREAM-TASK=b:321924870
TEST=run against ARCVM with change patch

Change-Id: I6cbdad495708fbf43e078a5fbda491b8424f1813
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/5479189
Reviewed-by: Yiwei Zhang <zzyiwei@chromium.org>
Commit-Queue: Dawn Han <dawnhan@google.com>
Tested-by: Dawn Han <dawnhan@google.com>
---
 arch/x86/kvm/vmx/vmx.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 81c101bbc1aa9d40bdfca50f4ee47c0b7c6bbb11..fa9a9b168c4bba50459d38e140df3298c25783e3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7658,6 +7658,9 @@ u8 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio,
 	if (is_mmio)
 		return MTRR_TYPE_UNCACHABLE << VMX_EPT_MT_EPTE_SHIFT;
 
+	if (slot->flags & KVM_MEM_NON_COHERENT_DMA)
+		return MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT;
+
 	/*
 	 * Force WB and ignore guest PAT if the VM does NOT have a non-coherent
 	 * device attached and the CPU doesn't support self-snoop.  Letting the
-- 
2.46.0.rc2.264.g509ed76dc8-goog