Re: [patch] kvm with mmu notifier v18

[Marcelo Tosatti <mtosatti@redhat.com>]

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.

The memslots are protected by memslots_lock, which is always taken when
mmu_guess_page_from_pte_write() is reached.

mmap_sem protects QEMU's virtual mmaping from changing during find_vma /
get_user_pages.

Can you explain please?