Re: [PATCH 5 of 5] kvm: powerpc: Map guest userspace with TID=0 mappings

[Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>]

On Monday 28 July 2008 12:33:41 Liu Yu wrote:
> I have a question that I could not think through.
> While multiple qemu/kvm processes are running at the same time, how to
> prevent one guest from using others' TLB? For all the guests have the
> same TID=0 for userspace and TID=1 for kernel.
[...]

Hi Yu Liu, thats a good question.
Afaik thats solved by the fact that the shadow tlb which is used when entering 
guest context is per vcpu. Therefor a guest has always it's own shadow tlb 
active and no mappings to the content of other guests.

This patch just allows us that a single guest userspace process accessing the 
kernel 20 times (and changing privilege level 20 times by doing so) can run 
without tlb flushes.
Guest-userspace context switch (pid is changing) -> tlb flush; and guest 
switches (guest A -> guest B) -> other shadow tlb active; should still be 
working fine.

> >
> > The net is that we don't need to flush the TLB on privilege
> > switches, but we do on guest context switches (which are far
> > more infrequent). Guest boot time performance improvement: about 30%.
> >

-- 

Grüsse / regards, 
Christian Ehrhardt
IBM Linux Technology Center, Open Virtualization