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    SubjectRe: [PATCH 5/6] kvm, x86: use ro page and don't copy shared page
    On 07/17/2010 07:26 AM, Marcelo Tosatti wrote:
    > On Fri, Jul 16, 2010 at 10:19:36AM +0300, Gleb Natapov wrote:
    >> On Fri, Jul 16, 2010 at 10:13:07AM +0800, Lai Jiangshan wrote:
    >>> When page fault, we always call get_user_pages(write=1).
    >>>
    >>> Actually, we don't need to do this when it is not write fault.
    >>> get_user_pages(write=1) will cause shared page(ksm) copied.
    >>> If this page is not modified in future, this copying and the copied page
    >>> are just wasted. Ksm may scan and merge them and may cause thrash.
    >>>
    >> But is page is written into afterwords we will get another page fault.
    >>
    >>> In this patch, if the page is RO for host VMM and it not write fault for guest,
    >>> we will use RO page, otherwise we use a writable page.
    >>>
    >> Currently pages allocated for guest memory are required to be RW, so after your series
    >> behaviour will remain exactly the same as before.
    >
    > Except KSM pages.
    >
    >>> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
    >>> ---
    >>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
    >>> index 8ba9b0d..6382140 100644
    >>> --- a/arch/x86/kvm/mmu.c
    >>> +++ b/arch/x86/kvm/mmu.c
    >>> @@ -1832,6 +1832,45 @@ static void kvm_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn)
    >>> }
    >>> }
    >>>
    >>> +/* get a current mapped page fast, and test whether the page is writable. */
    >>> +static struct page *get_user_page_and_protection(unsigned long addr,
    >>> + int *writable)
    >>> +{
    >>> + struct page *page[1];
    >>> +
    >>> + if (__get_user_pages_fast(addr, 1, 1, page) == 1) {
    >>> + *writable = 1;
    >>> + return page[0];
    >>> + }
    >>> + if (__get_user_pages_fast(addr, 1, 0, page) == 1) {
    >>> + *writable = 0;
    >>> + return page[0];
    >>> + }
    >>> + return NULL;
    >>> +}
    >>> +
    >>> +static pfn_t kvm_get_pfn_for_page_fault(struct kvm *kvm, gfn_t gfn,
    >>> + int write_fault, int *host_writable)
    >>> +{
    >>> + unsigned long addr;
    >>> + struct page *page;
    >>> +
    >>> + if (!write_fault) {
    >>> + addr = gfn_to_hva(kvm, gfn);
    >>> + if (kvm_is_error_hva(addr)) {
    >>> + get_page(bad_page);
    >>> + return page_to_pfn(bad_page);
    >>> + }
    >>> +
    >>> + page = get_user_page_and_protection(addr, host_writable);
    >>> + if (page)
    >>> + return page_to_pfn(page);
    >>> + }
    >>> +
    >>> + *host_writable = 1;
    >>> + return kvm_get_pfn_for_gfn(kvm, gfn);
    >>> +}
    >>> +
    >> kvm_get_pfn_for_gfn() returns fault_page if page is mapped RO, so caller
    >> of kvm_get_pfn_for_page_fault() and kvm_get_pfn_for_gfn() will get
    >> different results when called on the same page. Not good.
    >> kvm_get_pfn_for_page_fault() logic should be folded into
    >> kvm_get_pfn_for_gfn().
    >
    > Agreed. Please keep gfn_to_pfn related code in virt/kvm/kvm_main.c.
    >
    >

    Pass write_fault parameter to kvm_get_pfn_for_gfn()?
    But only X86 use this parameter currently, I think it is OK to
    keep these code in arch/x86/kvm/mmu.c


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