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Re: [Qemu-devel] 4G address space remapping on 64-bit host

From: Blue Swirl
Subject: Re: [Qemu-devel] 4G address space remapping on 64-bit host
Date: Tue, 3 Jul 2007 10:48:49 +0300

On 6/29/07, Fabrice Bellard <address@hidden> wrote:
The kqemu part could be quite simple. A new execution mode could be
added so that:

- shadow page table faults generate a specific signal in the user guest
- A kqemu "syscall" callable from the user guest code could be used to
do the equivalent of tlb_set_page(), tlb_flush_page() and tlb_flush().

Note that I don't think it is worth using Xen for that. Modifying kqemu
(or even KVM) should be more flexible. With kqemu it could also work on
FreeBSD, Solaris, Windows and Linux.

The more complicated part is to split QEMU in two parts : one part
containing the translator (and maybe some devices) would be executed as
guest user code in kqemu. The other part would be executed as a regular
process to handle what is left (graphic, disk access, etc).

The first step could be execution of TB code from kqemu, memory
accesses could bypass the TLB.

If TB where MMIO accesses are done are compiled specifically, I think it
can be quite efficient.

For the specific sparc32 case, I think that a better register window
handling and a faster soft mmu code (using 4MB TLBs as it was proposed
in a patch long ago) should already give an important speed boost (say a
factor 1.5 to 2). The kqemu optimisation should give at least as much
performance gain, depending on the ratio of instructions which do a
memory access and on the number of TLB faults.

About register windows, it's strange that enabling REG_REGWPTR
mysteriously does not work.

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