Re: Scheduling of VCPUs and allocation of Guest Physical Memory

[Sukanto Ghosh <sukanto@cse.iitb.ac.in>]

Anthony Liguori wrote:
> Sukanto Ghosh wrote:
>>>> ii) Who does the allocation of host physical memory to the guest, 
>>>> is it KVM module or the associated QEmu process ?
>>>
>>> The QEMU process allocates the memory via malloc().
>> Doesn't the QEMU process and the guest have different address-spaces 
>> ? So, how can it malloc for the guest ? I thought the QEMU process 
>> requests for the memory allocation to the kvm module via the /dev/kvm 
>> device node.
>
> 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.



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



Thanks and Regards,

Sukanto Ghosh