Fabrice Bellard wrote:
Some questions:
- QEMU already maintains modified page status for VGA memory (and
kqemu for example fully supports that), so I don't see why KVM needs a
new method.
KQEMU passes the dirty bitmap directly to the kernel. KVM does
aggressive shadow page table caching though so maintaining the bitmap
requires removing write protection from the shadow page table entries
explicitly whenever you want to reset it. This is not something you
would want to do every time you go back and forth between
userspace/kernelspace.
KVM also doesn't pass the phys_map to the kernel like KQEMU does.
Instead, it divides memory into a set of slots. slots are contiguous
areas of RAM memory. An IO access that does fall into a slot is treated
as MMIO and is then sent to userspace. We then use the phys_map in
userspace to dispatch the MMIO operation.
There are only a handful of slots and they happen to be arranged in
order of most frequent access (I believe) such that you can very quickly
determine whether memory is MMIO or not.
- Why is kvm_cpu_register_physical_memory() needed ? kqemu can work
without it because there is a remapping between physical memory and
RAM address. I suggest to add that feature in KVM or to modify
cpu_register_physical_memory() to hide it.
The only reason the second call exists is to simplify the backwards
compatibility code. I will fix it properly though because I do agree
with you that it shouldn't be necessary.
- If KVM implements its own CPU loop, why are there patches in
libqemu.a (CPU core) ?
Good question! I looked through the code and some of it was just dead
code from before we had our own main loop. The rest is as follows:
In exec.c, we need to bump the size of the phys_map to support larger
memory (since we use it to dispatch MMIO). We also need to ensure that
cpu_interrupt calls into KVM code. There are also hooks for debugging
support. We've added more flags to cpu.h that we use when synchronizing
KVM register state to CPUState. We also added some additional state to
CPUState that we need to use.
Other than that, I've removed everything else.
Regards,
Anthony Liguori
Regards,
Fabrice.
