On Oct 1, 2013, at 6:39 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > On Thu, Sep 05, 2013 at 06:29:06PM +0800, Xiao Guangrong wrote: >> Currently, kvm zaps the large spte if write-protected is needed, the later >> read can fault on that spte. Actually, we can make the large spte readonly >> instead of making them un-present, the page fault caused by read access can >> be avoided >> >> The idea is from Avi: >> | As I mentioned before, write-protecting a large spte is a good idea, >> | since it moves some work from protect-time to fault-time, so it reduces >> | jitter. This removes the need for the return value. >> >> This version has fixed the issue reported in 6b73a9606, the reason of that >> issue is that fast_page_fault() directly sets the readonly large spte to >> writable but only dirty the first page into the dirty-bitmap that means >> other pages are missed. Fixed it by only the normal sptes (on the >> PT_PAGE_TABLE_LEVEL level) can be fast fixed >> >> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx> >> --- >> arch/x86/kvm/mmu.c | 36 ++++++++++++++++++++---------------- >> arch/x86/kvm/x86.c | 8 ++++++-- >> 2 files changed, 26 insertions(+), 18 deletions(-) >> >> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c >> index 869f1db..88107ee 100644 >> --- a/arch/x86/kvm/mmu.c >> +++ b/arch/x86/kvm/mmu.c >> @@ -1177,8 +1177,7 @@ static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep) >> >> /* >> * Write-protect on the specified @sptep, @pt_protect indicates whether >> - * spte writ-protection is caused by protecting shadow page table. >> - * @flush indicates whether tlb need be flushed. >> + * spte write-protection is caused by protecting shadow page table. >> * >> * Note: write protection is difference between drity logging and spte >> * protection: >> @@ -1187,10 +1186,9 @@ static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep) >> * - for spte protection, the spte can be writable only after unsync-ing >> * shadow page. >> * >> - * Return true if the spte is dropped. >> + * Return true if tlb need be flushed. >> */ >> -static bool >> -spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush, bool pt_protect) >> +static bool spte_write_protect(struct kvm *kvm, u64 *sptep, bool pt_protect) >> { >> u64 spte = *sptep; >> >> @@ -1200,17 +1198,11 @@ spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush, bool pt_protect) >> >> rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep); >> >> - if (__drop_large_spte(kvm, sptep)) { >> - *flush |= true; >> - return true; >> - } >> - >> if (pt_protect) >> spte &= ~SPTE_MMU_WRITEABLE; >> spte = spte & ~PT_WRITABLE_MASK; >> >> - *flush |= mmu_spte_update(sptep, spte); >> - return false; >> + return mmu_spte_update(sptep, spte); >> } > > Is it necessary for kvm_mmu_unprotect_page to search for an entire range large > page range now, instead of a 4k page? It is unnecessary. kvm_mmu_unprotect_page is used to delete the gfn's shadow pages then vcpu will try to re-fault. If any gfn in the large range has shadow page, it will stop using large mapping, so that the mapping will be split to small mappings when vcpu re-fault again. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html