Re: [patch] kvm with mmu notifier v18

Andrea Arcangeli
2008-06-12T01:34:04Z

On Tue, Jun 10, 2008 at 05:41:30PM -0300, Marcelo Tosatti wrote:
> Hi Andrea,
>
> On Thu, Jun 05, 2008 at 06:47:17PM +0200, Andrea Arcangeli wrote:
> > latest version, I removed ->release already before posting the last
> > one because by the time vm destroy runs no more guest mode can run, so
> > sptes are irrelevant and no cpu can follow the secondary tlb anymore
> > because no cpu can be in guest mode for the 'mm', even if sptes are
> > actually destroyed later. The previous patch was taking a kvm mutex in
> > release under mmu_lock and that's forbidden, so it's simpler to remove
> > the release debug knob for now (you suggested to use
> > kvm_reload_remote_mmus in the future that shouldn't take sleeping
> > locks). The only reason for having a real ->release would be to avoid
> > any risk w.r.t. to tlb speculative accesses to gart alias with
> > different cache protocol (I doubt this is a realistic worry but anyway
> > it's not big deal to implement a ->release).
> >
> > Signed-off-by: Andrea Arcangeli <andrea@qumranet.com>
>
> > +static int mmu_guess_page_from_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
> > + const u8 *new, int bytes,
> > + gfn_t *_gfn, pfn_t *_pfn,
> > + int *_mmu_seq, int *_largepage)
> > {
> > gfn_t gfn;
> > int r;
> > u64 gpte = 0;
> > pfn_t pfn;
> > -
> > - vcpu->arch.update_pte.largepage = 0;
> > + int mmu_seq;
> > + int largepage;
> >
> > if (bytes != 4 && bytes != 8)
> > - return;
> > + return 0;
> >
> > /*
> > * Assume that the pte write on a page table of the same type
> > @@ -1650,7 +1770,7 @@ static void mmu_guess_page_from_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
> > if ((bytes == 4) && (gpa % 4 == 0)) {
> > r = kvm_read_guest(vcpu->kvm, gpa & ~(u64)7, &gpte, 8);
> > if (r)
> > - return;
> > + return 0;
> > memcpy((void *)&gpte + (gpa % 8), new, 4);
> > } else if ((bytes == 8) && (gpa % 8 == 0)) {
> > memcpy((void *)&gpte, new, 8);
> > @@ -1660,23 +1780,30 @@ static void mmu_guess_page_from_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
> > memcpy((void *)&gpte, new, 4);
> > }
> > if (!is_present_pte(gpte))
> > - return;
> > + return 0;
> > gfn = (gpte & PT64_BASE_ADDR_MASK) >> PAGE_SHIFT;
> >
> > + largepage = 0;
> > down_read(&current->mm->mmap_sem);
> > if (is_large_pte(gpte) && is_largepage_backed(vcpu, gfn)) {
> > gfn &= ~(KVM_PAGES_PER_HPAGE-1);
> > - vcpu->arch.update_pte.largepage = 1;
> > + largepage = 1;
> > }
> > + mmu_seq = atomic_read(&vcpu->kvm->arch.mmu_notifier_seq);
> > + /* implicit mb(), we'll read before PT lock is unlocked */
> > pfn = gfn_to_pfn(vcpu->kvm, gfn);
> > up_read(&current->mm->mmap_sem);
> >
> > - if (is_error_pfn(pfn)) {
> > + if (unlikely(is_error_pfn(pfn))) {
> > kvm_release_pfn_clean(pfn);
> > - return;
> > + return 0;
> > }
> > - vcpu->arch.update_pte.gfn = gfn;
> > - vcpu->arch.update_pte.pfn = pfn;
> > +
> > + *_gfn = gfn;
> > + *_pfn = pfn;
> > + *_mmu_seq = mmu_seq;
> > + *_largepage = largepage;
> > + return 1;
> > }
> >
> > static void kvm_mmu_access_page(struct kvm_vcpu *vcpu, gfn_t gfn)
> > @@ -1711,9 +1838,24 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
> > int npte;
> > int r;
> >
> > + int update_pte;
> > + gfn_t gpte_gfn;
> > + pfn_t pfn;
> > + int mmu_seq;
> > + int largepage;
> > +
> > pgprintk("%s: gpa %llx bytes %d\n", __func__, gpa, bytes);
> > - mmu_guess_page_from_pte_write(vcpu, gpa, new, bytes);
> > + update_pte = mmu_guess_page_from_pte_write(vcpu, gpa, new, bytes,
> > + &gpte_gfn, &pfn,
> > + &mmu_seq, &largepage);
> > spin_lock(&vcpu->kvm->mmu_lock);
> > + if (update_pte) {
> > + BUG_ON(!is_error_pfn(vcpu->arch.update_pte.pfn));
> > + vcpu->arch.update_pte.gfn = gpte_gfn;
> > + vcpu->arch.update_pte.pfn = pfn;
> > + vcpu->arch.update_pte.mmu_seq = mmu_seq;
> > + vcpu->arch.update_pte.largepage = largepage;
> > + }
>
> I don't get this. mmu_lock protects the shadow page tables, reverse
> mappings and associated lists. vcpu->arch.update_pte is a per-vcpu
> structure, so it does not need locking by itself.

Ok, I wasn't sure myself if this is needed. The question is if two
physical cpus could ever access vcpu->arch.update_pte structure at the
same time... I guess answer is no in which case I can freely undo the
above.