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

Jun Koi wrote:

On 6/26/07, Dor Laor [off-list ref] wrote:

quoted

...
+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

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