Re: [PATCH RFC 5/6] KVM: mark requests that do not need a wakeup

From: "Radim Krčmář" <rkrcmar@redhat.com>
To: Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	Christoffer Dall <cdall@linaro.org>,
	Andrew Jones <drjones@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Cornelia Huck <cornelia.huck@de.ibm.com>,
	James Hogan <james.hogan@imgtec.com>,
	Paul Mackerras <paulus@ozlabs.org>
Subject: Re: [PATCH RFC 5/6] KVM: mark requests that do not need a wakeup
Date: Fri, 7 Apr 2017 14:29:11 +0200	[thread overview]
Message-ID: <20170407122910.GD23559@potion> (raw)
In-Reply-To: <eab3a2dd-3ade-57f2-ec2b-1ebd744df0fa@arm.com>

2017-04-07 09:27+0100, Marc Zyngier:
> On 06/04/17 21:20, Radim Krčmář wrote:
>> Some operations must ensure that the guest is not running with stale
>> data, but if the guest is halted, then the update can wait until another
>> event happens.  kvm_make_all_requests() currently doesn't wake up, so we
>> can mark all requests used with it.
>> 
>> First 8 bits were arbitrarily reserved for request numbers.
>> 
>> Most uses of requests have the request type as a constant, so a compiler
>> will optimize the '&'.
>> 
>> An alternative would be to have an inline function that would return
>> whether the request needs a wake-up or not, but I like this one better
>> even though it might produce worse assembly.
>> 
>> Suggested-by: Christoffer Dall <cdall@linaro.org>
>> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
>> ---
>>   Btw. do you recall which macro allowed to define bitmasks?  (It has
>>   two arguments, FROM and TO.)
> 
> GENMASK (and its _ULL variant), defined in include/linux/bitops.h.

Thank you, it is under BIT() ... I am blind.

> +#define KVM_REQUEST_MASK           0xff

The 0xff should be "GENMASK(7,0)".

First 8 bits is plenty and should be fast even if the compiler doesn't
optimize the masking because request is not constant.