[PATCH RFC v1 03/12] KVM: x86/mmu: Prepare kvm_is_mmio_pfn() for PG_reserved changes

[David Hildenbrand <david@redhat.com>]

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 - however, there is no reliable and
fast check to detect memmaps that were initialized and are ZONE_DEVICE.

Let's rewrite kvm_is_mmio_pfn() so we really only touch initialized
memmaps that are guaranteed to not contain garbage. Make sure that
RAM without a memmap is still not detected as MMIO and that ZONE_DEVICE
that is not UC/UC-/WC is not detected as MMIO.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: KarimAllah Ahmed <karahmed@amazon.de>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 arch/x86/kvm/mmu.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 24c23c66b226..795869ffd4bb 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2962,20 +2962,26 @@ 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);
+
+	/*
+	 * Online pages consist of pages managed by the buddy. Especially,
+	 * ZONE_DEVICE pages are never online. Online pages that are reserved
+	 * indicate the zero page and MMIO pages.
+	 */
+	if (page)
+		return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn));
+
+	/*
+	 * Anything with a valid memmap could be ZONE_DEVICE - or the
+	 * memmap could be uninitialized. 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