Re: [PATCH v2 07/14] KVM: arm64/sve: Make register ioctl access errors more consistent

From: "Alex Bennée" <alex.bennee@linaro.org>
To: Dave Martin <Dave.Martin@arm.com>
Cc: Okamoto Takayuki <tokamoto@jp.fujitsu.com>,
	Christoffer Dall <cdall@kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Zhang Lei <zhang.lei@jp.fujitsu.com>,
	Julien Grall <julien.grall@arm.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 07/14] KVM: arm64/sve: Make register ioctl access errors more consistent
Date: Thu, 25 Apr 2019 16:04:36 +0100	[thread overview]
Message-ID: <87pnpaunwb.fsf@zen.linaroharston> (raw)
In-Reply-To: <20190425130424.GA3567@e103592.cambridge.arm.com>

Dave Martin <Dave.Martin@arm.com> writes:

> On Thu, Apr 25, 2019 at 01:30:29PM +0100, Alex Bennée wrote:
>>
>> Dave Martin <Dave.Martin@arm.com> writes:
>>
>> > Currently, the way error codes are generated when processing the
>> > SVE register access ioctls in a bit haphazard.
>> >
>> > This patch refactors the code so that the behaviour is more
>> > consistent: now, -EINVAL should be returned only for unrecognised
>> > register IDs or when some other runtime error occurs.  -ENOENT is
>> > returned for register IDs that are recognised, but whose
>> > corresponding register (or slice) does not exist for the vcpu.
>> >
>> > To this end, in {get,set}_sve_reg() we now delegate the
>> > vcpu_has_sve() check down into {get,set}_sve_vls() and
>> > sve_reg_to_region().  The KVM_REG_ARM64_SVE_VLS special case is
>> > picked off first, then sve_reg_to_region() plays the role of
>> > exhaustively validating or rejecting the register ID and (where
>> > accepted) computing the applicable register region as before.
>> >
>> > sve_reg_to_region() is rearranged so that -ENOENT or -EPERM is not
>> > returned prematurely, before checking whether reg->id is in a
>> > recognised range.
>> >
>> > -EPERM is now only returned when an attempt is made to access an
>> > actually existing register slice on an unfinalized vcpu.
>> >
>> > Fixes: e1c9c98345b3 ("KVM: arm64/sve: Add SVE support to register access ioctl interface")
>> > Fixes: 9033bba4b535 ("KVM: arm64/sve: Add pseudo-register for the guest's vector lengths")
>> > Suggested-by: Andrew Jones <drjones@redhat.com>
>> > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
>> > Reviewed-by: Andrew Jones <drjones@redhat.com>
>
> [...]
>
>> > @@ -335,25 +344,30 @@ static int sve_reg_to_region(struct sve_state_reg_region *region,
>> >  	/* Verify that we match the UAPI header: */
>> >  	BUILD_BUG_ON(SVE_NUM_SLICES != KVM_ARM64_SVE_MAX_SLICES);
>> >
>> > -	if ((reg->id & SVE_REG_SLICE_MASK) > 0)
>> > -		return -ENOENT;
>> > -
>> > -	vq = sve_vq_from_vl(vcpu->arch.sve_max_vl);
>> > -
>> >  	reg_num = (reg->id & SVE_REG_ID_MASK) >> SVE_REG_ID_SHIFT;
>> >
>> >  	if (reg->id >= zreg_id_min && reg->id <= zreg_id_max) {
>> > +		if (!vcpu_has_sve(vcpu) || (reg->id & SVE_REG_SLICE_MASK) > 0)
>> > +			return -ENOENT;
>> > +
>> > +		vq = sve_vq_from_vl(vcpu->arch.sve_max_vl);
>> > +
>> >  		reqoffset = SVE_SIG_ZREG_OFFSET(vq, reg_num) -
>> >  				SVE_SIG_REGS_OFFSET;
>> >  		reqlen = KVM_SVE_ZREG_SIZE;
>> >  		maxlen = SVE_SIG_ZREG_SIZE(vq);
>> >  	} else if (reg->id >= preg_id_min && reg->id <= preg_id_max) {
>> > +		if (!vcpu_has_sve(vcpu) || (reg->id & SVE_REG_SLICE_MASK) > 0)
>> > +			return -ENOENT;
>> > +
>> > +		vq = sve_vq_from_vl(vcpu->arch.sve_max_vl);
>> > +
>>
>> I suppose you could argue for a:
>>
>> 	if (reg->id >= zreg_id_min && reg->id <= preg_id_max) {
>> 		if (!vcpu_has_sve(vcpu) || (reg->id & SVE_REG_SLICE_MASK) > 0)
>> 			return -ENOENT;
>>
>> 		vq = sve_vq_from_vl(vcpu->arch.sve_max_vl);
>>
>>                 if (reg->id <= zreg_id_max) {
>> 			reqoffset = SVE_SIG_ZREG_OFFSET(vq, reg_num) -
>> 				SVE_SIG_REGS_OFFSET;
>> 			reqlen = KVM_SVE_ZREG_SIZE;
>> 			maxlen = SVE_SIG_ZREG_SIZE(vq);
>>                 } else {
>> 			reqoffset = SVE_SIG_PREG_OFFSET(vq, reg_num) -
>> 				SVE_SIG_REGS_OFFSET;
>> 			reqlen = KVM_SVE_PREG_SIZE;
>> 			maxlen = SVE_SIG_PREG_SIZE(vq);
>> 		}
>> 	} else {
>> 		return -EINVAL;
>> 	}
>>
>> but only for minimal DRY reasons.
>
> Agreed, but that bakes in another assumption: that the ZREG and PREG ID
> ranges are contiguous.

Ahh I'd misread:

  /* reg ID ranges for P- registers and FFR (which are contiguous) */

However these are defined in the UABI:

  /* Z- and P-regs occupy blocks at the following offsets within this range: */
  #define KVM_REG_ARM64_SVE_ZREG_BASE	0
  #define KVM_REG_ARM64_SVE_PREG_BASE	0x400
  #define KVM_REG_ARM64_SVE_FFR_BASE	0x600

so there position is pretty fixed now right?

> I preferred to keep the number of assumptions down.
>
> Althoug the resulting code wasn't ideal, the actual amount of
> duplication that I ended up with here seemed low enough as to be
> acceptable (though opinions can differ on that).

It's no biggie ;-)

--
Alex Bennée

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