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

Anthony Liguori
2007-06-26T11:57:27Z

Jun Koi wrote:
> On 6/26/07, Dor Laor <dor.laor-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:
>> ...
>> +static __init struct kvm_paravirt_state *paravirt_alloc_state(void)
>> +{
>> + struct kvm_paravirt_state *state;
>> +
>> + state = (void *)get_zeroed_page(GFP_KERNEL);
>> + if (!state)
>> + goto err;
>> +
>> + state->vmca = (void *)get_zeroed_page(GFP_KERNEL);
>> + if (!state->vmca)
>> + goto err;
>> +
>> + /* FIXME: what do I need for this to be executable on 64 bit? */
>> + state->hypercall = (void *)get_zeroed_page(GFP_KERNEL);
>>
>> Why do you alloc a page for the hypercall instead of using Ingo's code
>> below? This way it can work for 64 bit too.
>>
>> Ingo's code:
>> /*
>> * This is the vm-syscall address - to be patched by the host to
>> * VMCALL (Intel) or VMMCALL (AMD), depending on the CPU model:
>> */
>> asm (
>> " .globl hypercall_addr \n"
>> " .align 4 \n"
>> " hypercall_addr: \n"
>> " movl $-38, %eax \n"
>> " ret \n"
>> );
>
> The assembly code "movl $-38, %eax; \nret" is only a "reserved place",
> which is later overwritten by hypercall address from the host, isnt
> it?
>
> If so, why dont we simply put 4 NOPs there?

So if the hypervisor fails to patch it, we get a proper errno instead of
running off into random code.

Regards,

Anthony Liguori

> Thanks,
> Jun