Re: [PATCH] Add MCE support to KVM

[Avi Kivity <avi@redhat.com>]

Gerd Hoffmann wrote:
>> That said, I'd like to be able to emulate the Xen HVM hypercalls. But in
>> any case, they hypercall implementation has to be in the kernel,
>
> No.  With Xenner the xen hypercall emulation code lives in guest 
> address space.

In this case the guest ring-0 code should trap the #GP, and install the 
hypercall page (which uses sysenter/syscall?).  No kvm or qemu changes 
needed.

>> Especially if we need to support
>> tricky bits like continuations.
>
> Is there any reason to?  I *think* xen does it for better scheduling 
> latency.  But with xen emulation sitting in guest address space we can 
> schedule the guest at will anyway.

It also improves latency within the guest itself.  At least I think that 
what was the Hyper-V spec is saying.  You can interrupt the execution of 
a long hypercall, inject and interrupt, and resume.  Sort of like a 
rep/movs instruction, which the cpu can and will interrupt.

>>> Same MSR, multiple writes (page number in the low bits).
>>
>> Nasty. The hypervisor has to remember all of the pages, so it can update
>> them for live migration.
>
> Xenner doesn't need update-on-migration, so there is no need at all to 
> remember this.  At the end of the day it is just memcpy(guest, data, 
> PAGESIZE) triggered by wrmsr. 

For Xenner, no (and you don't need to intercept the msr at all), but for 
pv-on-hvm, you do need to update the code.

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