Subject: Re: [PATCH v8 03/28] arm64: mte: CPU feature detection and initial sysreg configuration

From: Marc Zyngier <[email-masked]>
Date: Tue, 25 Aug 2020 14:53:47 +0100

On 2020-08-25 11:54, Catalin Marinas wrote:
> On Tue, Aug 25, 2020 at 09:53:16AM +0100, Marc Zyngier wrote:
>> On 2020-08-24 19:27, Catalin Marinas wrote:
>> > diff --git a/arch/arm64/include/asm/kvm_arm.h
>> > b/arch/arm64/include/asm/kvm_arm.h
>> > index 8a1cbfd544d6..6c3b2fc922bb 100644
>> > --- a/arch/arm64/include/asm/kvm_arm.h
>> > +++ b/arch/arm64/include/asm/kvm_arm.h
>> > @@ -78,7 +78,7 @@
>> >  			 HCR_AMO | HCR_SWIO | HCR_TIDCP | HCR_RW | HCR_TLOR | \
>> >  			 HCR_FMO | HCR_IMO)
>> >  #define HCR_VIRT_EXCP_MASK (HCR_VSE | HCR_VI | HCR_VF)
>> > -#define HCR_HOST_NVHE_FLAGS (HCR_RW | HCR_API | HCR_APK)
>> > +#define HCR_HOST_NVHE_FLAGS (HCR_RW | HCR_API | HCR_APK | HCR_ATA)
>> >  #define HCR_HOST_VHE_FLAGS (HCR_RW | HCR_TGE | HCR_E2H)
>> 
>> Why is HCR_ATA only set for nVHE? HCR_EL2.ATA seems to apply to both,
>> doesn't it?
> 
> We need HCR_EL2.ATA to be set when !VHE so that the host kernel can use
> MTE. That said, I think we need to turn it off when running a guest.
> Even if we hide the ID register, the guest may still attempt to enable
> tags on some memory that doesn't support it, leading to unpredictable
> behaviour (well, only if we expose device memory to guests directly;
> Steve's patches will deal with this but for now we just disable MTE in
> guests).
> 
> With VHE, HCR_EL2.ATA only affects the guests, so it can stay off. The
> host's use of tags is controlled by SCTLR_EL1/EL2.ATA (i.e. HCR_EL2.ATA
> has no effect if E2H and TGE are both 1; qemu has a bug here which I
> discovered yesterday).

Ah, I missed that too.

> 
>> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> > index 077293b5115f..59b91f58efec 100644
>> > --- a/arch/arm64/kvm/sys_regs.c
>> > +++ b/arch/arm64/kvm/sys_regs.c
>> > @@ -1131,6 +1131,8 @@ static u64 read_id_reg(const struct kvm_vcpu
>> > *vcpu,
>> >  		if (!vcpu_has_sve(vcpu))
>> >  			val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
>> >  		val &= ~(0xfUL << ID_AA64PFR0_AMU_SHIFT);
>> > +	} else if (id == SYS_ID_AA64PFR1_EL1) {
>> > +		val &= ~(0xfUL << ID_AA64PFR1_MTE_SHIFT);
>> 
>> Hiding the capability is fine, but where is the handling of trapping
>> instructions done? They should result in an UNDEF being injected.
> 
> They are a few new MTE-specific MSR/MRS which are trapped at EL2 but
> since KVM doesn't understand them yet, shouldn't it already inject
> undef back at EL1? That would be safer regardless of MTE support.

An UNDEF will be injected, but not without spitting a nastygram in
the kernel log (look at emulate_sys_reg()).

The best course of action is to have an entry in the sysreg table
that would explicitly do the handling.

Thanks,

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

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