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

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Message-ID: <20170407122910.GD23559@potion>
Date:   Fri, 7 Apr 2017 14:29:11 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Christoffer Dall <cdall@...aro.org>,
        Andrew Jones <drjones@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cornelia.huck@...ibm.com>,
        James Hogan <james.hogan@...tec.com>,
        Paul Mackerras <paulus@...abs.org>
Subject: Re: [PATCH RFC 5/6] KVM: mark requests that do not need a wakeup

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@...aro.org>
>> Signed-off-by: Radim Krčmář <rkrcmar@...hat.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.