Re: [Qemu-ppc] [Qemu-devel] [PATCH] ppc: Stop dumping state on all exceptions in linux-user

[Richard Henderson]

On 08/03/2016 05:09 PM, Benjamin Herrenschmidt wrote:
> As far user-with-softmmu, I'm not too sure... softmmu significantly
> increases the overhead of load and stores. Maybe after we add 128-bit
> integers to TGC to alleviate that a bit ? :-)

It wouldn't be mandatory, but there are certain bugs we can't fix without it. 
The big issues to be fixed with softmmu are

(1) Host page size > guest page size.

E.g. there are many programs (i386, sparc, etc, all with 4k pages) that you 
can't even load, much less run, on a ppc64 host using a 64k page size.

(2) Host virtual address space bits != guest virtual address space bits

My alpha emulation has run into this.  A real Alpha guest has a 44-bit address 
space, but an x86_64 host has a 48-bit address space.  The x86_64 kernel cannot 
be persuaded to reliably map memory below (1ul << 44), so I have to pretend 
than Alpha has a 48-bit address space.  (Indeed, I set this to 63 bits, so that 
it works for even wider va, like on ppc64 and sparc64.)

More theoretically, if the guest uses high bits for some purpose (e.g. ia64 
segmentation in the top 3 bits), and the host doesn't have a full 64-bit 
virtual address space, then we cannot even map the program, since we cannot set 
bits 61-63 to non-zero values.


r~