linux-kernel - Re: [PATCH 06/12] KVM: x86: API changes for SMM support

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Message-ID: <20150521162658.GB7525@potion.brq.redhat.com>
Date:	Thu, 21 May 2015 18:26:58 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, bsd@...hat.com
Subject: Re: [PATCH 06/12] KVM: x86: API changes for SMM support

2015-05-21 16:59+0200, Paolo Bonzini:
> On 21/05/2015 16:49, Radim Krčmář wrote:
>> 2015-05-08 13:20+0200, Paolo Bonzini:
>>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>>> @@ -202,7 +202,7 @@ struct kvm_run {
>>>  	__u32 exit_reason;
>>>  	__u8 ready_for_interrupt_injection;
>>>  	__u8 if_flag;
>>> -	__u8 padding2[2];
>>> +	__u16 flags;
>> 
>> (It got lost last review and I'd really like to know ...
>>  what is the advantage of giving both bytes to flags?)
> 
> No advantage.  You just should leave padding2[1] in the middle so that
> the offset of &run->padding2[0] doesn't change.

I don't get that.  The position of padding should be decided by
comparing probabilities of extending 'if_flag' and 'flags'.

>                                                  Since it's not obvious
> I gave two bytes to flags, but I can do it either way.

if_flag seems to be set in stone as one bit, so I'd vote for

  __u8 flags;
  __u8 padding2;

(Or 'padding3', to prevent the same class of errors that removing it
 altogether does;  which we didn't do for other tailed padding).

For there isn't much space left in struct kvm ...
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