Re: [kvmarm] [PATCH 09/29] arm64: KVM: system register handling

Christoffer Dall
2013-04-23T23:07:06Z

On Mon, Mar 25, 2013 at 1:19 AM, Marc Zyngier <marc.zyngier@arm.com> wrote:
> Hi Alex,
>
> On Thu, 7 Mar 2013 11:30:20 +0100, Alexander Graf <agraf@suse.de> wrote:
>> On 05.03.2013, at 04:47, Marc Zyngier wrote:
>>
>>> Provide 64bit system register handling, modeled after the cp15
>>> handling for ARM.
>>>
>>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>>> ---
>
> [...]
>
>>> +static int emulate_sys_reg(struct kvm_vcpu *vcpu,
>>> + const struct sys_reg_params *params)
>>> +{
>>> + size_t num;
>>> + const struct sys_reg_desc *table, *r;
>>> +
>>> + table = get_target_table(vcpu->arch.target, &num);
>>> +
>>> + /* Search target-specific then generic table. */
>>> + r = find_reg(params, table, num);
>>> + if (!r)
>>> + r = find_reg(params, sys_reg_descs, ARRAY_SIZE(sys_reg_descs));
>>
>> Searching through the whole list sounds quite slow. Especially since the
>> TLS register is at the very bottom of it.
>>
>> Can't you make this a simple switch() statement through a bit of #define
>> and maybe #include magic? After all, the sysreg target encoding is all
> part
>> of the opcode. And from my experience in the PPC instruction emulator,
>> switch()es are _a lot_ faster than any other way of lookup I've tried.
>
> So I've had a go at implementing this, and decided it is not worth the
> effort if we want to preserve the same level of functionality (ONE_REG
> discovery, sanity checking at VM startup...).
>
> Granted, we would gain a faster trap handling. But look at what we're
> actually trapping, and how often this happens. Almost nothing, almost
> never. So, until shown that we spend too much time iterating over the
> sys_reg_desc array, I'll keep it simple.
>
> This is not to say that there's no optimization to be made. Quite the
> opposite! Just that this particular one seems a bit overkill.
>
> Anyway, thanks for pushing me into pondering this! :-)
>
totally late in the game here, but I saw you discussed this briefly on
IRC as well, and I completely agree with Marc here, we have much
bigger fish to fry. On all the measurements I did on the 32-bit side,
this doesn't even begin to show up on the radar.

vgic man, vgic!

-Christoffer