Subject: [PATCH v1 02/10] KVM: x86/mmu: Prepare kvm_is_mmio_pfn() for PG_reserved changes

From: David Hildenbrand <[email-masked]>
Date: Thu, 24 Oct 2019 14:09:30 +0200

Right now, ZONE_DEVICE memory is always set PG_reserved. We want to
change that.

KVM has this weird use case that you can map anything from /dev/mem
into the guest. pfn_valid() is not a reliable check whether the memmap
was initialized and can be touched. pfn_to_online_page() makes sure
that we have an initialized memmap (and don't have ZONE_DEVICE memory).

Rewrite kvm_is_mmio_pfn() to make sure the function produces the
same result once we stop setting ZONE_DEVICE pages PG_reserved.

Cc: Paolo Bonzini <[email-masked]>
Cc: "Radim Krčmář" <[email-masked]>
Cc: Sean Christopherson <[email-masked]>
Cc: Vitaly Kuznetsov <[email-masked]>
Cc: Wanpeng Li <[email-masked]>
Cc: Jim Mattson <[email-masked]>
Cc: Joerg Roedel <[email-masked]>
Cc: Thomas Gleixner <[email-masked]>
Cc: Ingo Molnar <[email-masked]>
Cc: Borislav Petkov <[email-masked]>
Cc: "H. Peter Anvin" <[email-masked]>
Cc: KarimAllah Ahmed <[email-masked]>
Cc: Michal Hocko <[email-masked]>
Cc: Dan Williams <[email-masked]>
Signed-off-by: David Hildenbrand <[email-masked]>
---
 arch/x86/kvm/mmu.c | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 24c23c66b226..f03089a336de 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2962,20 +2962,25 @@ static bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
 
 static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
 {
+	struct page *page = pfn_to_online_page(pfn);
+
+	/*
+	 * ZONE_DEVICE pages are never online. Online pages that are reserved
+	 * either indicate the zero page or MMIO pages.
+	 */
+	if (page)
+		return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn));
+
+	/*
+	 * Anything with a valid (but not online) memmap could be ZONE_DEVICE.
+	 * Treat only UC/UC-/WC pages as MMIO.
+	 */
 	if (pfn_valid(pfn))
-		return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn)) &&
-			/*
-			 * Some reserved pages, such as those from NVDIMM
-			 * DAX devices, are not for MMIO, and can be mapped
-			 * with cached memory type for better performance.
-			 * However, the above check misconceives those pages
-			 * as MMIO, and results in KVM mapping them with UC
-			 * memory type, which would hurt the performance.
-			 * Therefore, we check the host memory type in addition
-			 * and only treat UC/UC-/WC pages as MMIO.
-			 */
-			(!pat_enabled() || pat_pfn_immune_to_uc_mtrr(pfn));
+		return !pat_enabled() || pat_pfn_immune_to_uc_mtrr(pfn);
 
+	/*
+	 * Any RAM that has no memmap (e.g., mapped via /dev/mem) is not MMIO.
+	 */
 	return !e820__mapped_raw_any(pfn_to_hpa(pfn),
 				     pfn_to_hpa(pfn + 1) - 1,
 				     E820_TYPE_RAM);
-- 
2.21.0