Re: [PATCH stable-5.12.y backport 1/2] KVM: arm64: Commit pending PC adjustemnts before returning to userspace

[Zenghui Yu]

Hi Marc,

On 2021/6/1 19:44, Marc Zyngier wrote:
> Hi Zenghui,
>
> Thanks for having a go at the backport.
>
> On Tue, 01 Jun 2021 12:12:37 +0100,
> Zenghui Yu <yuzenghui@xxxxxxxxxx> wrote:
> > From: Marc Zyngier <maz@xxxxxxxxxx>
> >
> > commit 26778aaa134a9aefdf5dbaad904054d7be9d656d upstream.
> >
> > KVM currently updates PC (and the corresponding exception state)
> > using a two phase approach: first by setting a set of flags,
> > then by converting these flags into a state update when the vcpu
> > is about to enter the guest.
> >
> > However, this creates a disconnect with userspace if the vcpu thread
> > returns there with any exception/PC flag set. In this case, the exposed
> > context is wrong, as userspace doesn't have access to these flags
> > (they aren't architectural). It also means that these flags are
> > preserved across a reset, which isn't expected.
> >
> > To solve this problem, force an explicit synchronisation of the
> > exception state on vcpu exit to userspace. As an optimisation
> > for nVHE systems, only perform this when there is something pending.
> >
> > Reported-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> > Reviewed-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> > Reviewed-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> > Tested-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx # 5.11
> > [yuz: stable-5.12.y backport: add __KVM_HOST_SMCCC_FUNC___kvm_adjust_pc
> >  macro manually and keep it consistent with mainline]
>
> I'd rather you allocated a new number here, irrespective of what
> mainline has (rational below).
>
> > Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/kvm_asm.h   |  1 +
> >  arch/arm64/kvm/arm.c               | 11 +++++++++++
> >  arch/arm64/kvm/hyp/exception.c     |  4 ++--
> >  arch/arm64/kvm/hyp/nvhe/hyp-main.c |  8 ++++++++
> >  4 files changed, 22 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> > index a8578d650bb6..d7f769bb6c9c 100644
> > --- a/arch/arm64/include/asm/kvm_asm.h
> > +++ b/arch/arm64/include/asm/kvm_asm.h
> > @@ -57,6 +57,7 @@
> >  #define __KVM_HOST_SMCCC_FUNC___kvm_get_mdcr_el2		12
> >  #define __KVM_HOST_SMCCC_FUNC___vgic_v3_save_aprs		13
> >  #define __KVM_HOST_SMCCC_FUNC___vgic_v3_restore_aprs		14
> > +#define __KVM_HOST_SMCCC_FUNC___kvm_adjust_pc			21
>
> This is going to generate a larger than necessary host_hcall array in
> hyp/nvhe/hyp-main.c, which we're trying to keep tightly packed for
> obvious reasons.

It isn't obvious to me ;-). But this creates some invalid entries
(HVC handlers) in the host_hcall array, which is not good. I'll change
__KVM_HOST_SMCCC_FUNC___kvm_adjust_pc to 15. Thanks for your reminder.

> With this nit fixed:
>
> Reviewed-by: Marc Zyngier <maz@xxxxxxxxxx>

Thanks!

Zenghui