Re: [PATCH 8/8] KVM: SEV-ES: go over the sev_pio_data buffer in multiple passes if needed

Maxim Levitsky
2021-10-21T23:15:39Z

On Wed, 2021-10-13 at 12:56 -0400, Paolo Bonzini wrote:
> The PIO scratch buffer is larger than a single page, and therefore
> it is not possible to copy it in a single step to vcpu->arch/pio_data.
> Bound each call to emulator_pio_in/out to a single page; keep
> track of how many I/O operations are left in vcpu->arch.sev_pio_count,
> so that the operation can be restarted in the complete_userspace_io
> callback.
>
> For OUT, this means that the previous kvm_sev_es_outs implementation
> becomes an iterator of the loop, and we can consume the sev_pio_data
> buffer before leaving to userspace.
>
> For IN, instead, consuming the buffer and decreasing sev_pio_count
> is always done in the complete_userspace_io callback, because that
> is when the memcpy is done into sev_pio_data.
>
> Cc: stable@vger.kernel.org
> Fixes: 7ed9abfe8e9f ("KVM: SVM: Support string IO operations for an SEV-ES guest")
> Reported-by: Felix Wilhelm <fwilhelm@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/x86.c | 73 +++++++++++++++++++++++++--------
> 2 files changed, 57 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 6bed6c416c6c..5a0298aa56ba 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -703,6 +703,7 @@ struct kvm_vcpu_arch {
> struct kvm_pio_request pio;
> void *pio_data;
> void *sev_pio_data;
> + unsigned sev_pio_count;
>
> u8 event_exit_inst_len;
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a485e185ad00..09c1e64495d3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12378,38 +12378,76 @@ int kvm_sev_es_mmio_read(struct kvm_vcpu *vcpu, gpa_t gpa, unsigned int bytes,
> EXPORT_SYMBOL_GPL(kvm_sev_es_mmio_read);
>
> static int kvm_sev_es_outs(struct kvm_vcpu *vcpu, unsigned int size,
> - unsigned int port, unsigned int count)
> + unsigned int port);
> +
> +static int complete_sev_es_emulated_outs(struct kvm_vcpu *vcpu)
> {
> - int ret = emulator_pio_out(vcpu, size, port,
> - vcpu->arch.sev_pio_data, count);
> + vcpu->arch.pio.count = 0;
> + if (vcpu->arch.sev_pio_count)
> + return kvm_sev_es_outs(vcpu,
> + vcpu->arch.pio.size,
> + vcpu->arch.pio.port);
> + return 1;
> +}
> +
> +static int kvm_sev_es_outs(struct kvm_vcpu *vcpu, unsigned int size,
> + unsigned int port)
> +{
> + for (;;) {
> + unsigned int count =
> + min_t(unsigned int, PAGE_SIZE / size, vcpu->arch.sev_pio_count);
> + int ret = emulator_pio_out(vcpu, size, port, vcpu->arch.sev_pio_data, count);
> +
> + /* memcpy done already by emulator_pio_out. */
> + vcpu->arch.sev_pio_count -= count;
> + vcpu->arch.sev_pio_data += count * vcpu->arch.pio.size;
> + if (!ret)
> + break;
>
> - if (ret) {
> /* Emulation done by the kernel. */
> - return ret;
> + vcpu->arch.pio.count = 0;
> + if (!vcpu->arch.sev_pio_count)
> + return 1;
> }
>
> - vcpu->arch.pio.count = 0;
> + vcpu->arch.complete_userspace_io = complete_sev_es_emulated_outs;
> return 0;
> }
>
> -static int complete_sev_es_emulated_ins(struct kvm_vcpu *vcpu)
> +static int kvm_sev_es_ins(struct kvm_vcpu *vcpu, unsigned int size,
> + unsigned int port);
> +
> +static void __complete_sev_es_emulated_ins(struct kvm_vcpu *vcpu)
> {
> - memcpy(vcpu->arch.sev_pio_data, vcpu->arch.pio_data,
> - vcpu->arch.pio.count * vcpu->arch.pio.size);
> - vcpu->arch.pio.count = 0;
> + unsigned count = vcpu->arch.pio.count;
> + complete_emulator_pio_in(vcpu, vcpu->arch.sev_pio_data);
> + vcpu->arch.sev_pio_count -= count;
> + vcpu->arch.sev_pio_data += count * vcpu->arch.pio.size;
> +}
>
> +static int complete_sev_es_emulated_ins(struct kvm_vcpu *vcpu)
> +{
> + __complete_sev_es_emulated_ins(vcpu);
> + if (vcpu->arch.sev_pio_count)
> + return kvm_sev_es_ins(vcpu,
> + vcpu->arch.pio.size,
> + vcpu->arch.pio.port);
> return 1;
> }
>
> static int kvm_sev_es_ins(struct kvm_vcpu *vcpu, unsigned int size,
> - unsigned int port, unsigned int count)
> + unsigned int port)
> {
> - int ret = emulator_pio_in(vcpu, size, port,
> - vcpu->arch.sev_pio_data, count);
> + for (;;) {
> + unsigned int count =
> + min_t(unsigned int, PAGE_SIZE / size, vcpu->arch.sev_pio_count);
> + if (!__emulator_pio_in(vcpu, size, port, count))
> + break;
>
> - if (ret) {
> /* Emulation done by the kernel. */
> - return ret;
> + __complete_sev_es_emulated_ins(vcpu);
> + if (!vcpu->arch.sev_pio_count)
> + return 1;
> }
>
> vcpu->arch.complete_userspace_io = complete_sev_es_emulated_ins;
> @@ -12421,8 +12459,9 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
> int in)
> {
> vcpu->arch.sev_pio_data = data;
> - return in ? kvm_sev_es_ins(vcpu, size, port, count)
> - : kvm_sev_es_outs(vcpu, size, port, count);
> + vcpu->arch.sev_pio_count = count;
> + return in ? kvm_sev_es_ins(vcpu, size, port)
> + : kvm_sev_es_outs(vcpu, size, port);
> }
> EXPORT_SYMBOL_GPL(kvm_sev_es_string_io);
>

I might have missed something, but it looks OK to me.
i mostly checked how the code looks after applying this patch.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Note that I haven't run tested the patches as I don't currently test SEV-ES.

Best regards,
Maxim Levitsky