Re: [kvm-devel] [PATCH 1/3] KVM paravirt_ops infrastructure

Anthony Liguori
2007-05-31T01:15:33Z

Rusty Russell wrote:
> On Wed, 2007-05-30 at 09:52 -0500, Anthony Liguori wrote:
>
>> This patch adds the basic infrastructure for paravirtualizing a KVM
>> guest.
>>
>
> Hi Anthony!
>
> Nice patch, comments below.
>
>
>> Discovery of running under KVM is done by sharing a page of memory
>> between
>> the guest and host (initially through an MSR write).
>>
>
> I missed the shared page in this patch? If you are going to do that,
> perhaps putting the hypercall magic in that page is a good idea?
>

para_state is the shared page. The address is passed to the KVM via the
MSR (so it's a shared page owned by the guest).

>> +extern unsigned char hypercall_addr[4];
>>
>
> Perhaps in a header?
>
>
>> +asm (
>> + ".globl hypercall_addr\n"
>> + ".align 4\n"
>> + "hypercall_addr:\n"
>> + "movl $-38, %eax\n"
>> + "ret\n"
>> +);
>>
>
> I don't think we want the hypercall returning Linux error numbers, and
> magic numbers are bad too. ud2 here I think.
>

Yeah, you're not the first one to suggest this. The thing is, KVM
already has host-side support for a hypercall API. I didn't want to
change that unless I had to. However, based on the prior feedback re:
using CPUID, I will be changing it so I'll update this too.

>> + para_state->guest_version = KVM_PARA_API_VERSION;
>> + para_state->host_version = -1;
>> + para_state->size = sizeof(*para_state);
>> + para_state->ret = 0;
>> + para_state->hypercall_gpa = __pa(hypercall_addr);
>>
>
> Two versions, size *and* ret? This seems like overkill...
>

Yeah, I agree :-) I actually am not a huge fan of using version
numbers. I think I'm going to try the next patch using a single version
number and a feature bitmap. Some of the optimizations (like MMU
batching) don't make sense in a NPT/EPT environment but the guest
shouldn't have to be aware of that.

>> + if (wrmsr_safe(MSR_KVM_API_MAGIC, __pa(para_state), 0)) {
>> + printk(KERN_INFO "KVM guest: WRMSR probe failed.\n");
>> + return -ENOENT;
>> + }
>>
>
> How about printk(KERN_INFO "I am not a KVM guest\n");?
>
>
>> +static int __init kvm_guest_init(void)
>> +{
>> + int rc;
>> +
>> + rc = kvm_guest_register_para(smp_processor_id());
>> + if (rc) {
>> + printk(KERN_INFO "paravirt KVM unavailable\n");
>>
>
> Double-printk when KVM isn't detected seems overkill. Perhaps you could
> just fold this all into one function...
>

Already have.

> (Personal gripe: I consider a variable named "rc" to be an admission of
> semantic defeat... "err" would be better here...)
>

I'm not sure I agree that's one's better than the other. Although I
guess if (err) { reads a little better...

Regards,

Anthony Liguori

> Thanks!
> Rusty.
>
>
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