Re: [PATCH 1/5] KVM paravirt_ops core infrastructure

Anthony Liguori
2007-06-18T12:25:41Z

Avi Kivity wrote:
> Anthony Liguori wrote:
>
>
>> - nr = vcpu->regs[VCPU_REGS_RBX] & -1u;
>> - a0 = vcpu->regs[VCPU_REGS_RAX] & -1u;
>> + nr = vcpu->regs[VCPU_REGS_RAX] & -1u;
>> + a0 = vcpu->regs[VCPU_REGS_RBX] & -1u;
>
>
>> - * Each hypercall may have 0-6 parameters.
>> - *
>> - * 64-bit hypercall index is in RAX, goes from 0 to __NR_hypercalls-1
>> - *
>> - * 64-bit parameters 1-6 are in the standard gcc x86_64 calling
>> convention
>> - * order: RDI, RSI, RDX, RCX, R8, R9.
>> + * Each hypercall may have 0-4 parameters.
>> *
>> - * 32-bit index is EBX, parameters are: EAX, ECX, EDX, ESI, EDI, EBP.
>> - * (the first 3 are according to the gcc regparm calling convention)
>> + * 32-bit index is EAX, parameters are: EBX, ECX, EDX, ESI.
>
>
> What's the motivation for these changes?

If we're queuing hypercalls, then having 4 arguments verses 6 means that
we can queue 50% more hypercalls in a single page. Using all six
arguments clobbers all the GP registers in 32-bit mode too.

Regards,

Anthony Liguori