On Tue, Jul 30, 2013 at 09:02:01PM +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.
>
> [
> It has fixed the issue reported in 6b73a9606 by stopping fast page fault
> marking the large spte to writable
> ]
Xiao,
Can you please write a comment explaining why are the problems
with shadow vs large read-only sptes (can't recall anymore),
and then why it is now safe to do it.
Comments below.
> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> ---
> arch/x86/kvm/mmu.c | 36 +++++++++++++++++-------------------
> 1 file changed, 17 insertions(+), 19 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index cf163ca..35d4b50 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1181,8 +1181,7 @@ 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:
> @@ -1191,10 +1190,9 @@ 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;
>
> @@ -1204,17 +1202,11 @@ 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);
> }
>
> static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,
> @@ -1226,11 +1218,8 @@ static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,
>
> for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
> BUG_ON(!(*sptep & PT_PRESENT_MASK));
> - if (spte_write_protect(kvm, sptep, &flush, pt_protect)) {
> - sptep = rmap_get_first(*rmapp, &iter);
> - continue;
> - }
>
> + flush |= spte_write_protect(kvm, sptep, pt_protect);
> sptep = rmap_get_next(&iter);
> }
>
> @@ -2701,6 +2690,8 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t v, int write,
> break;
> }
>
> + drop_large_spte(vcpu, iterator.sptep);
> +
> if (!is_shadow_present_pte(*iterator.sptep)) {
> u64 base_addr = iterator.addr;
>
> @@ -2855,7 +2846,7 @@ fast_pf_fix_direct_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> * - false: let the real page fault path to fix it.
> */
> static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
> - u32 error_code)
> + u32 error_code, bool force_pt_level)
> {
> struct kvm_shadow_walk_iterator iterator;
> struct kvm_mmu_page *sp;
> @@ -2884,6 +2875,13 @@ static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
> goto exit;
>
> /*
> + * Can not map the large spte to writable if the page is dirty
> + * logged.
> + */
> + if (sp->role.level > PT_PAGE_TABLE_LEVEL && force_pt_level)
> + goto exit;
> +
It is not safe to derive slot->dirty_bitmap like this:
since dirty log is enabled via RCU update, "is dirty bitmap enabled"
info could be stale by the time you check it here via the parameter,
so you can instantiate a large spte (because force_pt_level == false),
while you should not.
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