Re: Scheduling of VCPUs and allocation of Guest Physical Memory

[Avi Kivity <avi@qumranet.com>]

Sukanto Ghosh wrote:
>>
>> Yes, the guest has a different address space from the host.  The QEMU 
>> process malloc()'s the physical memory for the guest, and tells KVM 
>> what the region is (via an ioctl to /dev/kvm).
>
>
> Why does the QEMU process needs to malloc() the physical memory for 
> the guest ? Why can't it be done by the kernel itself ? Is it because 
> the said pages will be sharable between the QEMU process and the 
> guest, which will aid the QEMU process while performing DMA.
>

Userspace allocation is done in order to allow flexibility in how memory 
is allocated.  Userspace can choose to allocate large pages, apply a 
numa policy to the memory, mlock() it, etc.

It also makes swapping simple, as Linux already knows how to swap 
userspace memory.

Also, s390 has to do it this way.

>
>
> Also, when and how are host-initiated virtual interrupts delivered?  I 
> guess, that it is done at the time of VM-entry. 

Yes.

> But what about the interrupts that need to be delivered immediately 
> and while the VM is executing (assume interrupts are enabled in the 
> guest). Is any kind of signal-based mechanism employed to force a 
> VM-exit ?
>

Sending a signal to a task that is executing guest code will force it to 
exit to userspace immediately.  The in-kernel interrupt controller 
emulation also forces guest exits by sending inter-processor interrupts.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.