linux-kernel - [PATCH v2] kvm: Map PFN-type memory regions as writable (if possible)

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Message-Id: <1516213136-12883-1-git-send-email-karahmed@amazon.de>
Date:   Wed, 17 Jan 2018 19:18:56 +0100
From:   KarimAllah Ahmed <karahmed@...zon.de>
To:     kvm@...r.kernel.org
Cc:     KarimAllah Ahmed <karahmed@...zon.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2] kvm: Map PFN-type memory regions as writable (if possible)

For EPT-violations that are triggered by a read, the pages are also mapped with
write permissions (if their memory region is also writable). That would avoid
getting yet another fault on the same page when a write occurs.

This optimization only happens when you have a "struct page" backing the memory
region. So also enable it for memory regions that do not have a "struct page".

Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Cc: kvm@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
Signed-off-by: KarimAllah Ahmed <karahmed@...zon.de>

---
v2:
- Move setting writable to hva_to_pfn_remapped
- Extend hva_to_pfn_remapped interface to accept writable as a parameter
---
 virt/kvm/kvm_main.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 97da45e..88702d5 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1438,7 +1438,8 @@ static bool vma_is_valid(struct vm_area_struct *vma, bool write_fault)
 
 static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 			       unsigned long addr, bool *async,
-			       bool write_fault, kvm_pfn_t *p_pfn)
+			       bool write_fault, bool *writable,
+			       kvm_pfn_t *p_pfn)
 {
 	unsigned long pfn;
 	int r;
@@ -1464,6 +1465,8 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 
 	}
 
+	if (writable)
+		*writable = true;
 
 	/*
 	 * Get a reference here because callers of *hva_to_pfn* and
@@ -1529,7 +1532,7 @@ static kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
 	if (vma == NULL)
 		pfn = KVM_PFN_ERR_FAULT;
 	else if (vma->vm_flags & (VM_IO | VM_PFNMAP)) {
-		r = hva_to_pfn_remapped(vma, addr, async, write_fault, &pfn);
+		r = hva_to_pfn_remapped(vma, addr, async, write_fault, writable, &pfn);
 		if (r == -EAGAIN)
 			goto retry;
 		if (r < 0)
-- 
2.7.4