From mboxrd@z Thu Jan  1 00:00:00 1970
From: Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [RFC 0/5] Making KVM_GET_ONE_REG/KVM_SET_ONE_REG generic.
Date: Sat, 01 Sep 2012 22:05:52 +0930
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Cc: Peter Maydell <peter.maydell@linaro.org>,
	Christoffer Dall <c.dall@virtualopensystems.com>,
	Alexander Graf <agraf@suse.de>, kvmarm@lists.cs.columbia.edu,
	kvm-devel <kvm@vger.kernel.org>
To: Avi Kivity <avi@redhat.com>,
	Rusty Russell <rusty.russell@linaro.org>
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Avi Kivity <avi@redhat.com> writes:
>> -Capability: basic
>> +Capability: KVM_CAP_REG_LIST
>>  Architectures: arm
>
> all

OK, I guess that's to be true in future.  Fixed.

>>  Type: vcpu ioctl
>> -Parameters: struct kvm_msr_list (in/out)
>> +Parameters: struct kvm_reg_list (in/out)
>>  Returns: 0 on success; -1 on error
>>  Errors:
>> -  E2BIG:     the msr index list is too big to fit in the array specified by
>> -             the user.
>> +  E2BIG:     the reg index list is too big to fit in the array specified by
>> +             the user (the number required will be written into n).
>>  
>>  struct kvm_msr_list {
>> -	__u32 nmsrs; /* number of msrs in entries */
>> -	__u32 indices[0];
>> +	__u64 n; /* number of registers in reg[] */
>> +	__u64 reg[0];
>>  };
>>  
>
> People complain that this interface is hard to use.
>
> How about supplying the address of the array (in addition to n) so you
> don't have to deal with variable sized arrays, and dropping E2BIG in
> favour of always updating n (n changed to something bigger than you had
> -> reallocate and rerun)

We re-write n anyway, *and* return -E2BIG.  Not returning an error is
asking for trouble.

Passing an address in a struct is pretty bad, since it involves
compatibility wrappers.  I don't think that is what makes the API hard
to use.

Cheers,
Rusty.