Subject: Re: [RFC PATCH v12 02/33] KVM: Use gfn instead of hva for mmu_notifier_retry

From: Binbin Wu <[email-masked]>
Date: Thu, 14 Sep 2023 11:07:12 +0800

On 9/14/2023 9:55 AM, Sean Christopherson wrote:
> From: Chao Peng <[email-masked]>
>
> Currently in mmu_notifier invalidate path, hva range is recorded and
> then checked against by mmu_notifier_retry_hva() in the page fault
> handling path. However, for the to be introduced private memory, a page
> fault may not have a hva associated, checking gfn(gpa) makes more sense.
>
> For existing hva based shared memory, gfn is expected to also work. The
> only downside is when aliasing multiple gfns to a single hva, the
> current algorithm of checking multiple ranges could result in a much
> larger range being rejected. Such aliasing should be uncommon, so the
> impact is expected small.
>
> Suggested-by: Sean Christopherson <[email-masked]>
> Signed-off-by: Chao Peng <[email-masked]>
> Reviewed-by: Fuad Tabba <[email-masked]>
> Tested-by: Fuad Tabba <[email-masked]>
> [sean: convert vmx_set_apic_access_page_addr() to gfn-based API]
> Signed-off-by: Sean Christopherson <[email-masked]>
> ---
>   arch/x86/kvm/mmu/mmu.c   | 10 ++++++----
>   arch/x86/kvm/vmx/vmx.c   | 11 +++++------
>   include/linux/kvm_host.h | 33 +++++++++++++++++++++------------
>   virt/kvm/kvm_main.c      | 40 +++++++++++++++++++++++++++++++---------
>   4 files changed, 63 insertions(+), 31 deletions(-)
>
[...]
>   
> -void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
> -			      unsigned long end)
> +void kvm_mmu_invalidate_begin(struct kvm *kvm)
>   {
> +	lockdep_assert_held_write(&kvm->mmu_lock);
>   	/*
>   	 * The count increase must become visible at unlock time as no
>   	 * spte can be established without taking the mmu_lock and
>   	 * count is also read inside the mmu_lock critical section.
>   	 */
>   	kvm->mmu_invalidate_in_progress++;
> +
> +	if (likely(kvm->mmu_invalidate_in_progress == 1))
> +		kvm->mmu_invalidate_range_start = INVALID_GPA;
> +}
> +
> +void kvm_mmu_invalidate_range_add(struct kvm *kvm, gfn_t start, gfn_t end)
> +{
> +	lockdep_assert_held_write(&kvm->mmu_lock);
> +
> +	WARN_ON_ONCE(!kvm->mmu_invalidate_in_progress);
> +
>   	if (likely(kvm->mmu_invalidate_in_progress == 1)) {
>   		kvm->mmu_invalidate_range_start = start;
>   		kvm->mmu_invalidate_range_end = end;
> @@ -771,6 +781,12 @@ void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
>   	}
>   }
>   
> +static bool kvm_mmu_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
> +{
> +	kvm_mmu_invalidate_range_add(kvm, range->start, range->end);
> +	return kvm_unmap_gfn_range(kvm, range);
> +}
> +
>   static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
>   					const struct mmu_notifier_range *range)
>   {
> @@ -778,7 +794,7 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
>   	const struct kvm_mmu_notifier_range hva_range = {
>   		.start		= range->start,
>   		.end		= range->end,
> -		.handler	= kvm_unmap_gfn_range,
> +		.handler	= kvm_mmu_unmap_gfn_range,
>   		.on_lock	= kvm_mmu_invalidate_begin,
>   		.on_unlock	= kvm_arch_guest_memory_reclaimed,
>   		.flush_on_ret	= true,
> @@ -817,8 +833,7 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
>   	return 0;
>   }
>   
> -void kvm_mmu_invalidate_end(struct kvm *kvm, unsigned long start,
> -			    unsigned long end)
> +void kvm_mmu_invalidate_end(struct kvm *kvm)
>   {
>   	/*
>   	 * This sequence increase will notify the kvm page fault that
> @@ -833,6 +848,13 @@ void kvm_mmu_invalidate_end(struct kvm *kvm, unsigned long start,
>   	 * in conjunction with the smp_rmb in mmu_invalidate_retry().
>   	 */
>   	kvm->mmu_invalidate_in_progress--;
> +
> +	/*
> +	 * Assert that at least one range must be added between start() and
> +	 * end().  Not adding a range isn't fatal, but it is a KVM bug.
> +	 */
> +	WARN_ON_ONCE(kvm->mmu_invalidate_in_progress &&
> +		     kvm->mmu_invalidate_range_start == INVALID_GPA);
Should the check happen before the decrease of 
kvm->mmu_invalidate_in_progress?
Otherwise, KVM calls kvm_mmu_invalidate_begin(), then 
kvm_mmu_invalidate_end()
the check will not take effect.

>   }
>   
>   static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,