Re: [PATCH v19 110/130] KVM: TDX: Handle TDX PV MMIO hypercall

Binbin Wu
2024-04-18T09:30:07Z

On 2/26/2024 4:26 PM, isaku.yamahata@intel.com wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
>
> Export kvm_io_bus_read and kvm_mmio tracepoint and wire up TDX PV MMIO
> hypercall to the KVM backend functions.
>
> kvm_io_bus_read/write() searches KVM device emulated in kernel of the given
> MMIO address and emulates the MMIO. As TDX PV MMIO also needs it, export
> kvm_io_bus_read(). kvm_io_bus_write() is already exported. TDX PV MMIO
> emulates some of MMIO itself. To add trace point consistently with x86
> kvm, export kvm_mmio tracepoint.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/vmx/tdx.c | 114 +++++++++++++++++++++++++++++++++++++++++
> arch/x86/kvm/x86.c | 1 +
> virt/kvm/kvm_main.c | 2 +
> 3 files changed, 117 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 55fc6cc6c816..389bb95d2af0 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -1217,6 +1217,118 @@ static int tdx_emulate_io(struct kvm_vcpu *vcpu)
> return ret;
> }
>
> +static int tdx_complete_mmio(struct kvm_vcpu *vcpu)
> +{
> + unsigned long val = 0;
> + gpa_t gpa;
> + int size;
> +
> + KVM_BUG_ON(vcpu->mmio_needed != 1, vcpu->kvm);
> + vcpu->mmio_needed = 0;
> +
> + if (!vcpu->mmio_is_write) {
> + gpa = vcpu->mmio_fragments[0].gpa;
> + size = vcpu->mmio_fragments[0].len;
> +
> + memcpy(&val, vcpu->run->mmio.data, size);
> + tdvmcall_set_return_val(vcpu, val);
> + trace_kvm_mmio(KVM_TRACE_MMIO_READ, size, gpa, &val);
> + }
> + return 1;
> +}
> +
> +static inline int tdx_mmio_write(struct kvm_vcpu *vcpu, gpa_t gpa, int size,
> + unsigned long val)
> +{
> + if (kvm_iodevice_write(vcpu, &vcpu->arch.apic->dev, gpa, size, &val) &&
> + kvm_io_bus_write(vcpu, KVM_MMIO_BUS, gpa, size, &val))
> + return -EOPNOTSUPP;
> +
> + trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, size, gpa, &val);
> + return 0;
> +}
> +
> +static inline int tdx_mmio_read(struct kvm_vcpu *vcpu, gpa_t gpa, int size)
> +{
> + unsigned long val;
> +
> + if (kvm_iodevice_read(vcpu, &vcpu->arch.apic->dev, gpa, size, &val) &&
> + kvm_io_bus_read(vcpu, KVM_MMIO_BUS, gpa, size, &val))
> + return -EOPNOTSUPP;
> +
> + tdvmcall_set_return_val(vcpu, val);
> + trace_kvm_mmio(KVM_TRACE_MMIO_READ, size, gpa, &val);
> + return 0;
> +}
> +
> +static int tdx_emulate_mmio(struct kvm_vcpu *vcpu)
> +{
> + struct kvm_memory_slot *slot;
> + int size, write, r;
> + unsigned long val;
> + gpa_t gpa;
> +
> + KVM_BUG_ON(vcpu->mmio_needed, vcpu->kvm);
> +
> + size = tdvmcall_a0_read(vcpu);
> + write = tdvmcall_a1_read(vcpu);
> + gpa = tdvmcall_a2_read(vcpu);
> + val = write ? tdvmcall_a3_read(vcpu) : 0;
> +
> + if (size != 1 && size != 2 && size != 4 && size != 8)
> + goto error;
> + if (write != 0 && write != 1)
> + goto error;
> +
> + /* Strip the shared bit, allow MMIO with and without it set. */
Based on the discussion
https://lore.kernel.org/all/ZcUO5sFEAIH68JIA@google.com/
Do we still allow the MMIO without shared bit?

> + gpa = gpa & ~gfn_to_gpa(kvm_gfn_shared_mask(vcpu->kvm));
> +
> + if (size > 8u || ((gpa + size - 1) ^ gpa) & PAGE_MASK)
"size > 8u" can be removed, since based on the check of size above, it
can't be greater than 8.

> + goto error;
> +
> + slot = kvm_vcpu_gfn_to_memslot(vcpu, gpa_to_gfn(gpa));
> + if (slot && !(slot->flags & KVM_MEMSLOT_INVALID))
> + goto error;
> +
> + if (!kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
Should this be checked for write first?

I check the handle_ept_misconfig() in VMX, it doesn't check write first
neither.

Functionally, it should be OK since guest will not read the address
range of fast mmio.
So the read case will be filtered out by ioeventfd_write().
But it has take a long way to get to ioeventfd_write().
Isn't it more efficient to check write first?

> + trace_kvm_fast_mmio(gpa);
> + return 1;
> + }
> +
> + if (write)
> + r = tdx_mmio_write(vcpu, gpa, size, val);
> + else
> + r = tdx_mmio_read(vcpu, gpa, size);
> + if (!r) {
> + /* Kernel completed device emulation. */
> + tdvmcall_set_return_code(vcpu, TDVMCALL_SUCCESS);
> + return 1;
> + }
> +
> + /* Request the device emulation to userspace device model. */
> + vcpu->mmio_needed = 1;
> + vcpu->mmio_is_write = write;
> + vcpu->arch.complete_userspace_io = tdx_complete_mmio;
> +
> + vcpu->run->mmio.phys_addr = gpa;
> + vcpu->run->mmio.len = size;
> + vcpu->run->mmio.is_write = write;
> + vcpu->run->exit_reason = KVM_EXIT_MMIO;
> +
> + if (write) {
> + memcpy(vcpu->run->mmio.data, &val, size);
> + } else {
> + vcpu->mmio_fragments[0].gpa = gpa;
> + vcpu->mmio_fragments[0].len = size;
> + trace_kvm_mmio(KVM_TRACE_MMIO_READ_UNSATISFIED, size, gpa, NULL);
> + }
> + return 0;
> +
> +error:
> + tdvmcall_set_return_code(vcpu, TDVMCALL_INVALID_OPERAND);
> + return 1;
> +}
> +
> static int handle_tdvmcall(struct kvm_vcpu *vcpu)
> {
> if (tdvmcall_exit_type(vcpu))
> @@ -1229,6 +1341,8 @@ static int handle_tdvmcall(struct kvm_vcpu *vcpu)
> return tdx_emulate_hlt(vcpu);
> case EXIT_REASON_IO_INSTRUCTION:
> return tdx_emulate_io(vcpu);
> + case EXIT_REASON_EPT_VIOLATION:
> + return tdx_emulate_mmio(vcpu);
> default:
> break;
> }
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 03950368d8db..d5b18cad9dcd 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -13975,6 +13975,7 @@ EXPORT_SYMBOL_GPL(kvm_sev_es_string_io);
>
> EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_entry);
> EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_exit);
> +EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_mmio);
> EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_fast_mmio);
> EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_inj_virq);
> EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_page_fault);
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index e27c22449d85..bc14e1f2610c 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2689,6 +2689,7 @@ struct kvm_memory_slot *kvm_vcpu_gfn_to_memslot(struct kvm_vcpu *vcpu, gfn_t gfn
>
> return NULL;
> }
> +EXPORT_SYMBOL_GPL(kvm_vcpu_gfn_to_memslot);
>
> bool kvm_is_visible_gfn(struct kvm *kvm, gfn_t gfn)
> {
> @@ -5992,6 +5993,7 @@ int kvm_io_bus_read(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr,
> r = __kvm_io_bus_read(vcpu, bus, &range, val);
> return r < 0 ? r : 0;
> }
> +EXPORT_SYMBOL_GPL(kvm_io_bus_read);
>
> int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
> int len, struct kvm_io_device *dev)