Re: [PATCH 00/16] x86: indirect call overhead reduction

[Julien Grall <julien.grall@arm.com>]

Hi Jan,

On 13/07/18 15:27, Jan Beulich wrote:
>>>> On 13.07.18 at 15:39, <julien.grall@arm.com> wrote:
>> On 13/07/18 14:27, Jan Beulich wrote:
>>>>>> On 13.07.18 at 15:00, <julien.grall@arm.com> wrote:
>>>> What would be the generic interface here? I saw it was based on
>>>> alternative for the plumbing.
>>>
>>> Yes, I'd prefer to use the same mechanism as presented in the series.
>>> As per above for the IOMMU case we'd then need another abstraction
>>> layer put in the middle (to produce a patch site on x86, but a normal
>>> [indirect] call on ARM).
>>
>> I will have a look. Could you point to the patch adding the abstraction?
> 
> Patch 9 is where the x86 side infrastructure gets added. Typical uses
> would then be (taken from patch 10)
> 
>   hvm_guest_x86_mode(struct vcpu *v)
>   {
>       ASSERT(v == current);
> -    return hvm_funcs.guest_x86_mode(v);
> +    return alternative_call1(hvm_funcs.guest_x86_mode, v);
OOI, is there any reason for you to define alternative_callN and not 
providing a generic version (similar to arm_smccc_1_1_smc in 
include/asm-arm/smccc.h)?

>   }
>   
>   static inline void
>   hvm_update_host_cr3(struct vcpu *v)
>   {
>       if ( hvm_funcs.update_host_cr3 )
> -        hvm_funcs.update_host_cr3(v);
> +        alternative_vcall1(hvm_funcs.update_host_cr3, v);
>   }
> 
> And actually there was one more question I meant to ask: Are all
> locations within Xen reachable by direct calls on ARM? I.e. can an
> indirect call, when we know its target, always be replaced, no
> matter at what address (within Xen) it lives?

For direct branch, the range supported is +/- 32MB for 32-bit Arm, +/- 
128MB for 64-bit Arm. With the current layout all Xen text (including 
livepatch payload) will always be below 12MB.

So we should be able to reach all Xen by direct calls.

Cheers,

-- 
Julien Grall

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