Re: [PATCH] RISC-V: KVM: Redirect instruction access fault trap to guest

[Anup Patel]

On Fri, Sep 13, 2024 at 6:09 AM Samuel Holland
<samuel.holland@xxxxxxxxxx> wrote:
>
> On 2024-09-12 4:03 AM, Quan Zhou wrote:
> >
> > On 2024/8/29 14:20, zhouquan@xxxxxxxxxxx wrote:
> >> From: Quan Zhou <zhouquan@xxxxxxxxxxx>
> >>
> >> The M-mode redirects an unhandled instruction access
> >> fault trap back to S-mode when not delegating it to
> >> VS-mode(hedeleg). However, KVM running in HS-mode
> >> terminates the VS-mode software when back from M-mode.
> >>
> >> The KVM should redirect the trap back to VS-mode, and
> >> let VS-mode trap handler decide the next step.
> >>
> >> Signed-off-by: Quan Zhou <zhouquan@xxxxxxxxxxx>
> >> ---
> >>   arch/riscv/kvm/vcpu_exit.c | 1 +
> >>   1 file changed, 1 insertion(+)
> >>
> >> diff --git a/arch/riscv/kvm/vcpu_exit.c b/arch/riscv/kvm/vcpu_exit.c
> >> index fa98e5c024b2..696b62850d0b 100644
> >> --- a/arch/riscv/kvm/vcpu_exit.c
> >> +++ b/arch/riscv/kvm/vcpu_exit.c
> >> @@ -182,6 +182,7 @@ int kvm_riscv_vcpu_exit(struct kvm_vcpu *vcpu, struct
> >> kvm_run *run,
> >>       ret = -EFAULT;
> >>       run->exit_reason = KVM_EXIT_UNKNOWN;
> >>       switch (trap->scause) {
> >> +    case EXC_INST_ACCESS:
> >
> > A gentle ping, the instruction access fault should be redirected to
> > VS-mode for handling, is my understanding correct?
>
> Yes, this looks correct. However, I believe it would be equivalent (and more
> efficient) to add EXC_INST_ACCESS to KVM_HEDELEG_DEFAULT in asm/kvm_host.h.
>
> I don't understand why some exceptions are delegated with hedeleg and others are
> caught and redirected here with no further processing. Maybe someone thought
> that it wasn't valid to set a bit in hedeleg if the corresponding bit was
> cleared in medeleg? But this doesn't make sense, as S-mode cannot know which
> bits are set in medeleg (maybe none are!).
>
> So the hypervisor must either:
>  1) assume M-mode firmware checks hedeleg and redirects exceptions to VS-mode
>     regardless of medeleg, in which case all four of these exceptions can be
>     moved to KVM_HEDELEG_DEFAULT and removed from this switch statement, or
>
>  2) assume M-mode might not check hedeleg and redirect exceptions to VS-mode,
>     and since no bits are guaranteed to be set in medeleg, any bit set in
>     hedeleg must _also_ be handled in the switch case here.
>
> Anup, Atish, thoughts?

Any exception delegated to VS-mode via hedeleg means it is directly delivered
to VS-mode without any intervention of HS-mode. This aligns with the RISC-V
priv specification and there is no alternate semantics assumed by KVM RISC-V.

At the moment, for KVM RISC-V we are converging towards the following
approach:

1) Only delegate "supervisor expected" traps to VS-mode via hedeleg
which supervisor software is generally expected to directly handle such
as breakpoint, user syscall, inst page fault, load page fault, and store
page fault.

2) Other "supervisor unexpected" traps are redirected to VS-mode via
software in HS-mode because these are not typically expected by supervisor
software and KVM RISC-V should at least gather some stats for such traps.
Previously, we were redirecting such unexpect traps to KVM user space
where the KVM user space tool will simply dump the VCPU state and kill
the Guest/VM.

The inst misaligned trap was historically always set in hedeleg but we
should update it based on the above approach.

>
> Regards,
> Samuel
>
> >
> >>       case EXC_INST_ILLEGAL:
> >>       case EXC_LOAD_MISALIGNED:
> >>       case EXC_STORE_MISALIGNED:
> >>
> >> base-commit: 7c626ce4bae1ac14f60076d00eafe71af30450ba
> >
> >
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>

Regards,
Anup