Re: [PATCH v7 12/14] KVM: Handle page fault for private memory

[Sean Christopherson]

On Wed, Jul 06, 2022, Chao Peng wrote:
> A page fault can carry the private/shared information for
> KVM_MEM_PRIVATE memslot, this can be filled by architecture code(like
> TDX code). To handle page fault for such access, KVM maps the page only
> when this private property matches the host's view on the page.
> 
> For a successful match, private pfn is obtained with memfile_notifier
> callbacks from private fd and shared pfn is obtained with existing
> get_user_pages.
> 
> For a failed match, KVM causes a KVM_EXIT_MEMORY_FAULT exit to
> userspace. Userspace then can convert memory between private/shared from
> host's view then retry the access.
> 
> Co-developed-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c          | 60 ++++++++++++++++++++++++++++++++-
>  arch/x86/kvm/mmu/mmu_internal.h | 18 ++++++++++
>  arch/x86/kvm/mmu/mmutrace.h     |  1 +
>  include/linux/kvm_host.h        | 35 ++++++++++++++++++-
>  4 files changed, 112 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 545eb74305fe..27dbdd4fe8d1 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3004,6 +3004,9 @@ int kvm_mmu_max_mapping_level(struct kvm *kvm,
>  	if (max_level == PG_LEVEL_4K)
>  		return PG_LEVEL_4K;
>  
> +	if (kvm_mem_is_private(kvm, gfn))
> +		return max_level;
> +
>  	host_level = host_pfn_mapping_level(kvm, gfn, pfn, slot);
>  	return min(host_level, max_level);
>  }
> @@ -4101,10 +4104,52 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
>  	kvm_mmu_do_page_fault(vcpu, work->cr2_or_gpa, 0, true);
>  }
>  
> +static inline u8 order_to_level(int order)
> +{
> +	enum pg_level level;
> +
> +	for (level = KVM_MAX_HUGEPAGE_LEVEL; level > PG_LEVEL_4K; level--)

Curly braces needed for the for-loop.

And I think it makes sense to take in the fault->max_level, that way this is
slightly more performant when the guest mapping is smaller than the host, e.g.

	for (level = max_level; level > PG_LEVEL_4K; level--)
		...

	return level;

Though I think I'd vote to avoid a loop entirely and do:

	BUILD_BUG_ON(KVM_MAX_HUGEPAGE_LEVEL > PG_LEVEL_1G);

	if (order > ???)
		return PG_LEVEL_1G;
	
	if (order > ???)
		return PG_LEVEL_2M;

	return PG_LEVEL_4K;


> +		if (order >= page_level_shift(level) - PAGE_SHIFT)
> +			return level;
> +	return level;
> +}
> +
> +static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
> +				   struct kvm_page_fault *fault)
> +{
> +	int order;
> +	struct kvm_memory_slot *slot = fault->slot;
> +	bool private_exist = kvm_mem_is_private(vcpu->kvm, fault->gfn);
> +
> +	if (fault->is_private != private_exist) {
> +		vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
> +		if (fault->is_private)
> +			vcpu->run->memory.flags = KVM_MEMORY_EXIT_FLAG_PRIVATE;
> +		else
> +			vcpu->run->memory.flags = 0;
> +		vcpu->run->memory.padding = 0;
> +		vcpu->run->memory.gpa = fault->gfn << PAGE_SHIFT;
> +		vcpu->run->memory.size = PAGE_SIZE;
> +		return RET_PF_USER;
> +	}
> +
> +	if (fault->is_private) {
> +		if (kvm_private_mem_get_pfn(slot, fault->gfn, &fault->pfn, &order))
> +			return RET_PF_RETRY;
> +		fault->max_level = min(order_to_level(order), fault->max_level);
> +		fault->map_writable = !(slot->flags & KVM_MEM_READONLY);
> +		return RET_PF_FIXED;
> +	}
> +
> +	/* Fault is shared, fallthrough. */
> +	return RET_PF_CONTINUE;
> +}
> +
>  static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  {
>  	struct kvm_memory_slot *slot = fault->slot;
>  	bool async;
> +	int r;
>  
>  	/*
>  	 * Retry the page fault if the gfn hit a memslot that is being deleted
> @@ -4133,6 +4178,12 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  			return RET_PF_EMULATE;
>  	}
>  
> +	if (kvm_slot_can_be_private(slot)) {
> +		r = kvm_faultin_pfn_private(vcpu, fault);
> +		if (r != RET_PF_CONTINUE)
> +			return r == RET_PF_FIXED ? RET_PF_CONTINUE : r;

I apologize if I've given you conflicting feedback in the past.  Now that this
returns RET_PF_* directly, I definitely think it makes sense to do:

	if (kvm_slot_can_be_private(slot) &&
	    fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
		vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
		if (fault->is_private)
			vcpu->run->memory.flags = KVM_MEMORY_EXIT_FLAG_PRIVATE;
		else
			vcpu->run->memory.flags = 0;
		vcpu->run->memory.padding = 0;
		vcpu->run->memory.gpa = fault->gfn << PAGE_SHIFT;
		vcpu->run->memory.size = PAGE_SIZE;
		return RET_PF_USER;
	}

	if (fault->is_private)
		return kvm_faultin_pfn_private(vcpu, fault);

That way kvm_faultin_pfn_private() only handles private faults, and this doesn't
need to play games with RET_PF_FIXED.


> +	}
> +
>  	async = false;
>  	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, &async,
>  					  fault->write, &fault->map_writable,
> @@ -4241,7 +4292,11 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>  		read_unlock(&vcpu->kvm->mmu_lock);
>  	else
>  		write_unlock(&vcpu->kvm->mmu_lock);
> -	kvm_release_pfn_clean(fault->pfn);
> +
> +	if (fault->is_private)
> +		kvm_private_mem_put_pfn(fault->slot, fault->pfn);
> +	else
> +		kvm_release_pfn_clean(fault->pfn);

AFAIK, we never bottomed out on whether or not this is needed[*].  Can you follow
up with Kirill to get an answer before posting v8?

[*] https://lore.kernel.org/all/20220620141647.GC2016793@xxxxxxxxxxxxxxxxxx