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

Sean Christopherson
2022-07-29T20:58:50Z

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@linux.intel.com>
> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
> 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@chaop.bj.intel.com