Re: [PATCH v11 08/13] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS hypercall
From: Ashish Kalra
Date: Tue Apr 06 2021 - 12:08:09 EST
On Tue, Apr 06, 2021 at 03:48:20PM +0000, Sean Christopherson wrote:
> On Mon, Apr 05, 2021, Ashish Kalra wrote:
> > From: Ashish Kalra <ashish.kalra@xxxxxxx>
>
> ...
>
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 3768819693e5..78284ebbbee7 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1352,6 +1352,8 @@ struct kvm_x86_ops {
> > int (*complete_emulated_msr)(struct kvm_vcpu *vcpu, int err);
> >
> > void (*vcpu_deliver_sipi_vector)(struct kvm_vcpu *vcpu, u8 vector);
> > + int (*page_enc_status_hc)(struct kvm_vcpu *vcpu, unsigned long gpa,
> > + unsigned long sz, unsigned long mode);
> > };
> >
> > struct kvm_x86_nested_ops {
> > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > index c9795a22e502..fb3a315e5827 100644
> > --- a/arch/x86/kvm/svm/sev.c
> > +++ b/arch/x86/kvm/svm/sev.c
> > @@ -1544,6 +1544,67 @@ static int sev_receive_finish(struct kvm *kvm, struct kvm_sev_cmd *argp)
> > return ret;
> > }
> >
> > +static int sev_complete_userspace_page_enc_status_hc(struct kvm_vcpu *vcpu)
> > +{
> > + vcpu->run->exit_reason = 0;
> > + kvm_rax_write(vcpu, vcpu->run->dma_sharing.ret);
> > + ++vcpu->stat.hypercalls;
> > + return kvm_skip_emulated_instruction(vcpu);
> > +}
> > +
> > +int svm_page_enc_status_hc(struct kvm_vcpu *vcpu, unsigned long gpa,
> > + unsigned long npages, unsigned long enc)
> > +{
> > + kvm_pfn_t pfn_start, pfn_end;
> > + struct kvm *kvm = vcpu->kvm;
> > + gfn_t gfn_start, gfn_end;
> > +
> > + if (!sev_guest(kvm))
> > + return -EINVAL;
> > +
> > + if (!npages)
> > + return 0;
>
> Parth of me thinks passing a zero size should be an error not a nop. Either way
> works, just feels a bit weird to allow this to be a nop.
>
> > +
> > + gfn_start = gpa_to_gfn(gpa);
>
> This should check that @gpa is aligned.
>
> > + gfn_end = gfn_start + npages;
> > +
> > + /* out of bound access error check */
> > + if (gfn_end <= gfn_start)
> > + return -EINVAL;
> > +
> > + /* lets make sure that gpa exist in our memslot */
> > + pfn_start = gfn_to_pfn(kvm, gfn_start);
> > + pfn_end = gfn_to_pfn(kvm, gfn_end);
> > +
> > + if (is_error_noslot_pfn(pfn_start) && !is_noslot_pfn(pfn_start)) {
> > + /*
> > + * Allow guest MMIO range(s) to be added
> > + * to the shared pages list.
> > + */
> > + return -EINVAL;
> > + }
> > +
> > + if (is_error_noslot_pfn(pfn_end) && !is_noslot_pfn(pfn_end)) {
> > + /*
> > + * Allow guest MMIO range(s) to be added
> > + * to the shared pages list.
> > + */
> > + return -EINVAL;
> > + }
>
> I don't think KVM should do any checks beyond gfn_end <= gfn_start. Just punt
> to userspace and give userspace full say over what is/isn't legal.
>
> > +
> > + if (enc)
> > + vcpu->run->exit_reason = KVM_EXIT_DMA_UNSHARE;
> > + else
> > + vcpu->run->exit_reason = KVM_EXIT_DMA_SHARE;
>
> Use a single exit and pass "enc" via kvm_run. I also strongly dislike "DMA",
> there's no guarantee the guest is sharing memory for DMA.
>
> I think we can usurp KVM_EXIT_HYPERCALL for this? E.g.
>
I see the following in Documentation/virt/kvm/api.rst :
..
..
/* KVM_EXIT_HYPERCALL */
struct {
__u64 nr;
__u64 args[6];
__u64 ret;
__u32 longmode;
__u32 pad;
} hypercall;
Unused. This was once used for 'hypercall to userspace'. To implement
such functionality, use KVM_EXIT_IO (x86) or KVM_EXIT_MMIO (all except s390).
This mentions this exitcode to be unused and implementing this
functionality using KVM_EXIT_IO for x86?
Thanks,
Ashish
> vcpu->run->exit_reason = KVM_EXIT_HYPERCALL;
> vcpu->run->hypercall.nr = KVM_HC_PAGE_ENC_STATUS;
> vcpu->run->hypercall.args[0] = gfn_start << PAGE_SHIFT;
> vcpu->run->hypercall.args[1] = npages * PAGE_SIZE;
> vcpu->run->hypercall.args[2] = enc;
> vcpu->run->hypercall.longmode = is_64_bit_mode(vcpu);
>
> > +
> > + vcpu->run->dma_sharing.addr = gfn_start;
>
> Addresses and pfns are not the same thing. If you're passing the size in bytes,
> then it's probably best to pass the gpa, not the gfn. Same for the params from
> the guest, they should be in the same "domain".
>
> > + vcpu->run->dma_sharing.len = npages * PAGE_SIZE;
> > + vcpu->arch.complete_userspace_io =
> > + sev_complete_userspace_page_enc_status_hc;
>
> I vote to drop the "userspace" part, it's already quite verbose.
>
> vcpu->arch.complete_userspace_io = sev_complete_page_enc_status_hc;
>
> > +
> > + return 0;
> > +}
> > +
>
> ..
>
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index f7d12fca397b..ef5c77d59651 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -8273,6 +8273,18 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> > kvm_sched_yield(vcpu->kvm, a0);
> > ret = 0;
> > break;
> > + case KVM_HC_PAGE_ENC_STATUS: {
> > + int r;
> > +
> > + ret = -KVM_ENOSYS;
> > + if (kvm_x86_ops.page_enc_status_hc) {
> > + r = kvm_x86_ops.page_enc_status_hc(vcpu, a0, a1, a2);
>
> Use static_call().
>
> > + if (r >= 0)
> > + return r;
> > + ret = r;
> > + }
>
> Hmm, an alternative to adding a kvm_x86_ops hook would be to tag the VM as
> supporting/allowing the hypercall. That would clean up this code, ensure VMX
> and SVM don't end up creating a different userspace ABI, and make it easier to
> reuse the hypercall in the future (I'm still hopeful :-) ). E.g.
>
> case KVM_HC_PAGE_ENC_STATUS: {
> u64 gpa = a0, nr_bytes = a1;
>
> if (!vcpu->kvm->arch.page_enc_hc_enable)
> break;
>
> if (!PAGE_ALIGNED(gpa) || !PAGE_ALIGNED(nr_bytes) ||
> !nr_bytes || gpa + nr_bytes <= gpa)) {
> ret = -EINVAL;
> break;
> }
>
> vcpu->run->exit_reason = KVM_EXIT_HYPERCALL;
> vcpu->run->hypercall.nr = KVM_HC_PAGE_ENC_STATUS;
> vcpu->run->hypercall.args[0] = gpa;
> vcpu->run->hypercall.args[1] = nr_bytes;
> vcpu->run->hypercall.args[2] = enc;
> vcpu->run->hypercall.longmode = op_64_bit;
> vcpu->arch.complete_userspace_io = complete_page_enc_hc;
> return 0;
> }
>
>
> > + break;
> > + }
> > default:
> > ret = -KVM_ENOSYS;
> > break;