Re: [PATCH v3] KVM: arm64: ARM_SMCCC_ARCH_WORKAROUND_1 doesn't return SMCCC_RET_NOT_REQUIRED

[Marc Zyngier]

On 2020-10-26 13:25, Will Deacon wrote:
> On Fri, Oct 23, 2020 at 08:47:50AM -0700, Stephen Boyd wrote:
> > According to the SMCCC spec[1](7.5.2 Discovery) the
> > ARM_SMCCC_ARCH_WORKAROUND_1 function id only returns 0, 1, and
> > SMCCC_RET_NOT_SUPPORTED.
> >
> >  0 is "workaround required and safe to call this function"
> >  1 is "workaround not required but safe to call this function"
> >  SMCCC_RET_NOT_SUPPORTED is "might be vulnerable or might not be, who 
> > knows, I give up!"
> >
> > SMCCC_RET_NOT_SUPPORTED might as well mean "workaround required, 
> > except
> > calling this function may not work because it isn't implemented in 
> > some
> > cases". Wonderful. We map this SMC call to
> >
> >  0 is SPECTRE_MITIGATED
> >  1 is SPECTRE_UNAFFECTED
> >  SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE
> >
> > For KVM hypercalls (hvc), we've implemented this function id to return
> > SMCCC_RET_NOT_SUPPORTED, 0, and SMCCC_RET_NOT_REQUIRED. One of those
> > isn't supposed to be there. Per the code we call
> > arm64_get_spectre_v2_state() to figure out what to return for this
> > feature discovery call.
> >
> >  0 is SPECTRE_MITIGATED
> >  SMCCC_RET_NOT_REQUIRED is SPECTRE_UNAFFECTED
> >  SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE
> >
> > Let's clean this up so that KVM tells the guest this mapping:
> >
> >  0 is SPECTRE_MITIGATED
> >  1 is SPECTRE_UNAFFECTED
> >  SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE
> >
> > Note: SMCCC_RET_NOT_AFFECTED is 1 but isn't part of the SMCCC spec
> >
> > Cc: Andre Przywara <andre.przywara@xxxxxxx>
> > Cc: Steven Price <steven.price@xxxxxxx>
> > Cc: Marc Zyngier <maz@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Link: https://developer.arm.com/documentation/den0028/latest [1]
> > Fixes: c118bbb52743 ("arm64: KVM: Propagate full Spectre v2 workaround 
> > state to KVM guests")
> > Signed-off-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
> > ---
> >
> > I see that before commit c118bbb52743 ("arm64: KVM: Propagate full
> > Spectre v2 workaround state to KVM guests") we had this mapping:
> >
> >  0 is SPECTRE_MITIGATED
> >  SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE
> >
> > so the return value '1' wasn't there then. Once the commit was merged 
> > we
> > introduced the notion of NOT_REQUIRED here when it shouldn't have been
> > introduced.
> >
> > Changes from v2:
> >  * Moved define to header file and used it
> >
> > Changes from v1:
> >  * Way longer commit text, more background (sorry)
> >  * Dropped proton-pack part because it was wrong
> >  * Rebased onto other patch accepted upstream
> >
> >  arch/arm64/kernel/proton-pack.c | 2 --
> >  arch/arm64/kvm/hypercalls.c     | 2 +-
> >  include/linux/arm-smccc.h       | 2 ++
> >  3 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/proton-pack.c 
> > b/arch/arm64/kernel/proton-pack.c
> > index 25f3c80b5ffe..c18eb7d41274 100644
> > --- a/arch/arm64/kernel/proton-pack.c
> > +++ b/arch/arm64/kernel/proton-pack.c
> > @@ -135,8 +135,6 @@ static enum mitigation_state 
> > spectre_v2_get_cpu_hw_mitigation_state(void)
> >  	return SPECTRE_VULNERABLE;
> >  }
> >
> > -#define SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED	(1)
> > -
> >  static enum mitigation_state 
> > spectre_v2_get_cpu_fw_mitigation_state(void)
> >  {
> >  	int ret;
> > diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> > index 9824025ccc5c..25ea4ecb6449 100644
> > --- a/arch/arm64/kvm/hypercalls.c
> > +++ b/arch/arm64/kvm/hypercalls.c
> > @@ -31,7 +31,7 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
> >  				val = SMCCC_RET_SUCCESS;
> >  				break;
> >  			case SPECTRE_UNAFFECTED:
> > -				val = SMCCC_RET_NOT_REQUIRED;
> > +				val = SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED;
> >  				break;
> >  			}
> >  			break;
> > diff --git a/include/linux/arm-smccc.h b/include/linux/arm-smccc.h
> > index 15c706fb0a37..0e50ba3e88d7 100644
> > --- a/include/linux/arm-smccc.h
> > +++ b/include/linux/arm-smccc.h
> > @@ -86,6 +86,8 @@
> >  			   ARM_SMCCC_SMC_32,				\
> >  			   0, 0x7fff)
> >
> > +#define SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED	1
>
> I thought we'd stick this in asm/spectre.h, but here is also good:
>
> Acked-by: Will Deacon <will@xxxxxxxxxx>

Acked-by: Marc Zyngier <maz@xxxxxxxxxx>

Will, if you're about to send fixes to Linus, can you please pick
this one up?

        M.
--
Jazz is not dead. It just smells funny...