Re: [Qemu-devel] [PATCH, RFC] Update qemu-tech.texi, needs verification

[Paul Brook <paul@codesourcery.com>]

> -@item QEMU can either use a full software MMU for maximum portability or
> use the host system call mmap() to simulate the target MMU. +@item
> +QEMU can either use a full software MMU for maximum portability or use
> +an in-kernel accelerator (kqemu) to simulate the target MMU. 

Referring to kqemu as a MMU simulator is at best very misleading. The item you 
removed was referring to qemu-fast, which (in principle at least) still 
worked for cross emulation.

kqemu and kvm execute [some of] the guest code natively, while continuing to 
emulate the rest of the machine.

> Various 
> +hardware devices can be emulated and in some cases, host devices
> +(e.g. serial and parallel ports, USB, drives) can be used
> +transparently by the guest Operating System for maximum performance.

This should be a seaprate item. As written it's unclear whether this is a 
kqemu feature or available all the time.

Host device passthrough is generally used for talking to external physical 
peripherals (e.g. a webcam, modem or tape drive), and not for performance 
reasons.

>  the condition codes are not needed by the next instructions, no
>  condition codes are computed at all.
>
> +This optimization is not yet implemented on other targets.

I don't think this back propagation pass exists at all now. It was made 
redundant by the TCG liveness pass.

The lazy condition code evaluation is used on x86, m68k and cris. ARM uses a 
simplified variant for the N and Z flags.

You might consider rewording the initial paragraph to say that lazy flag 
evaluation is important for CPUs where every instruction sets the condition 
codes.  It tends to be less important on conventional RISC systems where 
condition codes are only updated when explicitly requested.


Paul