Re: [PATCH v13 34/85] KVM: Get writable mapping for __kvm_vcpu_map() only when necessary
From: Sean Christopherson <seanjc@google.com>
Date: 2024-10-21 18:13:10
Also in:
kvm, kvm-riscv, kvmarm, linux-arm-kernel, linux-mips, linux-riscv, lkml, loongarch
On Mon, Oct 21, 2024, Yan Zhao wrote:
On Thu, Oct 10, 2024 at 11:23:36AM -0700, Sean Christopherson wrote:quoted
When creating a memory map for read, don't request a writable pfn from the primary MMU. While creating read-only mappings can be theoretically slower, as they don't play nice with fast GUP due to the need to break CoW before mapping the underlying PFN, practically speaking, creating a mapping isn't a super hot path, and getting a writable mapping for reading is weird and confusing. Tested-by: Alex Bennée <redacted> Signed-off-by: Sean Christopherson <seanjc@google.com> --- virt/kvm/kvm_main.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 080740f65061..b845e9252633 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c@@ -3122,7 +3122,7 @@ int __kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map, struct kvm_follow_pfn kfp = { .slot = gfn_to_memslot(vcpu->kvm, gfn), .gfn = gfn, - .flags = FOLL_WRITE, + .flags = writable ? FOLL_WRITE : 0, .refcounted_page = &map->pinned_page, .pin = true, };When writable is false, could we set ".pin = false," ?
Hmm, maybe? I can't imagine anything would actually break, but unless FOLL_PIN implies writing, my preference would still be to pin the page so that KVM always pins when accessing the actual data of a page.
Also not sure if ".map_writable = NULL" is missing.
Doh, my previous response was slightly wrong, it's implicitly initialized to NULL, not false. I forgot map_writable is a pointer to a bool.