[PATCH v2 16/18] KVM: arm64: Introduce __pkvm_tlb_flush_vmid()

[Fuad Tabba]

Hi Quentin,

On Wed, 11 Dec 2024 at 10:03, Quentin Perret <qperret at google.com> wrote:
>
> On Tuesday 10 Dec 2024 at 15:23:02 (+0000), Fuad Tabba wrote:
> > Hi Quentin,
> >
> > On Tue, 3 Dec 2024 at 10:38, Quentin Perret <qperret at google.com> wrote:
> > >
> > > Introduce a new hypercall to flush the TLBs of non-protected guests. The
> > > host kernel will be responsible for issuing this hypercall after changing
> > > stage-2 permissions using the __pkvm_host_relax_guest_perms() or
> > > __pkvm_host_wrprotect_guest() paths. This is left under the host's
> > > responsibility for performance reasons.
> > >
> > > Note however that the TLB maintenance for all *unmap* operations still
> > > remains entirely under the hypervisor's responsibility for security
> > > reasons -- an unmapped page may be donated to another entity, so a stale
> > > TLB entry could be used to leak private data.
> > >
> > > Signed-off-by: Quentin Perret <qperret at google.com>
> > > ---
> > >  arch/arm64/include/asm/kvm_asm.h   |  1 +
> > >  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 17 +++++++++++++++++
> > >  2 files changed, 18 insertions(+)
> > >
> > > diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> > > index 6178e12a0dbc..df6237d0459c 100644
> > > --- a/arch/arm64/include/asm/kvm_asm.h
> > > +++ b/arch/arm64/include/asm/kvm_asm.h
> > > @@ -87,6 +87,7 @@ enum __kvm_host_smccc_func {
> > >         __KVM_HOST_SMCCC_FUNC___pkvm_teardown_vm,
> > >         __KVM_HOST_SMCCC_FUNC___pkvm_vcpu_load,
> > >         __KVM_HOST_SMCCC_FUNC___pkvm_vcpu_put,
> > > +       __KVM_HOST_SMCCC_FUNC___pkvm_tlb_flush_vmid,
> > >  };
> > >
> > >  #define DECLARE_KVM_VHE_SYM(sym)       extern char sym[]
> > > diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > index de0012a75827..219d7fb850ec 100644
> > > --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > @@ -398,6 +398,22 @@ static void handle___kvm_tlb_flush_vmid(struct kvm_cpu_context *host_ctxt)
> > >         __kvm_tlb_flush_vmid(kern_hyp_va(mmu));
> > >  }
> > >
> > > +static void handle___pkvm_tlb_flush_vmid(struct kvm_cpu_context *host_ctxt)
> > > +{
> > > +       DECLARE_REG(pkvm_handle_t, handle, host_ctxt, 1);
> > > +       struct pkvm_hyp_vm *hyp_vm;
> > > +
> > > +       if (!is_protected_kvm_enabled())
> > > +               return;
> > > +
> > > +       hyp_vm = get_pkvm_hyp_vm(handle);
> > > +       if (!hyp_vm)
> > > +               return;
> > > +
> > > +       __kvm_tlb_flush_vmid(&hyp_vm->kvm.arch.mmu);
> > > +       put_pkvm_hyp_vm(hyp_vm);
> > > +}
> >
> > Since this is practically the same as kvm_tlb_flush_vmid(), does it
> > make sense to modify that instead (handle___kvm_tlb_flush_vmid()) to
> > do the right thing depending on whether pkvm is enabled? Thinking as
> > well for the future in case we want to support the rest of the
> > kvm_tlb_flush_vmid_*().
>
> I considered it, but the two implementations want different arguments --
> pkvm wants the handle while standard KVM uses the kvm struct address
> directly. I had an implementation at some point that multiplexed the
> implementations on a single HVC (we'd interpret the arguments
> differently depending on pKVM being enabled or not) but that felt more
> error prone than simply having two HVCs.
>
> Happy to reconsider if we can find a good way to make it work though.

I don't have a strong opinion about this. I think that for now, since
it's only this function, it's probably fine. That said, the
multiplexing is (as of patch 18, which I haven't reviewed yet) is just
lifted higher up to the host kernel, albeit with fewer parameters to
wiggle around.

To summarize, I think we can worry about it if/once we need the other
tlb_flush_* variants.

Cheers,
/fuad