Re: [PATCH 1/6] kvm.git mmu notifier patch

Marcelo Tosatti
2008-07-28T21:13:50Z

Hi Andrea,

On Fri, Jul 25, 2008 at 04:24:52PM +0200, Andrea Arcangeli wrote:
> Hello,
>
> This is the latest version of the kvm mmu notifier patch.
>
> This allows largepages to be invalidated too, and a separate patch
> takes care of alias locking (2/5) and memslot locking (3/5).
>
> While in the long term we should remove the alias to microoptimize the
> fast paths and avoid the need of unalias_gfn calls, I decided not to
> remove alias immediately: that patch done best would have removed the
> alias API as a whole from userland, and done bad it would have kept
> the alias API intact while implementing the lowlevel kernel details
> with memslots instead of the alias array. Problem is the memslot array
> is already fragmented with the private entries, and so adding the
> alias entries before or after the private memslot entries resulted in
> different issues, often requiring a new loop over the alias range or
> the private range depending where I put the alias slots (before or
> after the private ones).
>
> So I think to do it right the alias ioctl should be removed as a whole
> and that was too large to do right now, especially given we're
> concentrating on stability and fixing the alias with the mmu_lock was
> a 2 liner change.
>
> Marcelo please double check this code, I didn't effectively test it
> myself with the largepages yet but it looks the rmap_next works the
> same and the rmap_pde is just a rmapp with the only difference of the
> PSE bit being set. I didn't hide the rmap_pde details with gfn_to_rmap
> to avoid an entirely superflous gfn_to_memslot.
>
> + retval |= handler(kvm,
> + &memslot->lpage_info[
> + gfn_offset / KVM_PAGES_PER_HPAGE].rmap_pde);
>
> Signed-off-by: Andrea Arcangeli <andrea@qumranet.com>
>
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index 8d45fab..ce3251c 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -21,6 +21,7 @@ config KVM
> tristate "Kernel-based Virtual Machine (KVM) support"
> depends on HAVE_KVM
> select PREEMPT_NOTIFIERS
> + select MMU_NOTIFIER
> select ANON_INODES
> ---help---
> Support hosting fully virtualized guest machines using hardware
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 2b60b7d..9bc31fc 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -653,6 +653,84 @@ static void rmap_write_protect(struct kvm *kvm, u64 gfn)
> account_shadowed(kvm, gfn);
> }
>
> +static int kvm_unmap_rmapp(struct kvm *kvm, unsigned long *rmapp)
> +{
> + u64 *spte;
> + int need_tlb_flush = 0;
> +
> + while ((spte = rmap_next(kvm, rmapp, NULL))) {
> + BUG_ON(!(*spte & PT_PRESENT_MASK));
> + rmap_printk("kvm_rmap_unmap_hva: spte %p %llx\n", spte, *spte);
> + rmap_remove(kvm, spte);
> + set_shadow_pte(spte, shadow_trap_nonpresent_pte);
> + need_tlb_flush = 1;
> + }

Is rmap_remove() safe during rmap_next() iteration? Don't you need to
restart the rmap walk from the beginning?

Remember the discussion around commit
6597ca09e6c0e5aec7ffd2b8ab48c671d3c28414.

> + return need_tlb_flush;
> +}
> +
> +static int kvm_handle_hva(struct kvm *kvm, unsigned long hva,
> + int (*handler)(struct kvm *kvm, unsigned long *rmapp))
> +{
> + int i;
> + int retval = 0;
> +
> + /*
> + * If mmap_sem isn't taken, we can look the memslots with only
> + * the mmu_lock by skipping over the slots with userspace_addr == 0.
> + */
> + for (i = 0; i < kvm->nmemslots; i++) {
> + struct kvm_memory_slot *memslot = &kvm->memslots[i];
> + unsigned long start = memslot->userspace_addr;
> + unsigned long end;
> +
> + /* mmu_lock protects userspace_addr */
> + if (!start)
> + continue;
> +
> + end = start + (memslot->npages << PAGE_SHIFT);
> + if (hva >= start && hva < end) {
> + gfn_t gfn_offset = (hva - start) >> PAGE_SHIFT;
> + retval |= handler(kvm, &memslot->rmap[gfn_offset]);
> + retval |= handler(kvm,
> + &memslot->lpage_info[
> + gfn_offset /
> + KVM_PAGES_PER_HPAGE].rmap_pde);

Other than that looks good.