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Re: [Qemu-devel] Re: Compile files only once: some planning
From: |
Jamie Lokier |
Subject: |
Re: [Qemu-devel] Re: Compile files only once: some planning |
Date: |
Thu, 25 Mar 2010 03:08:44 +0000 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
Anthony Liguori wrote:
> It's a statement of correctness really. Devices should never deal with
> target_phys_addr_t's.
Shouldn't they? On real hardware, 64-bit PCI devices switch to being
32-bit PCI when plugged into a 32-bit motherboard slot.
> The question is, should a pci_addr_t or a sysbus_addr_t be 64 bit or
> should it be 32-bit on 32-bit platforms.
Depends what you want to emulate. It's not an accurate emulation if
all the old PCI devices provide 64-bit BARs; that could conceivably
bite some old OS, which expects NE2000-PCI to be a 32-bit device, for example.
And it's not a repeatable emulation if switching beteen 32-bit and
64-bit hosts means the guest sees a change in the PCI devices. It
should be possible to change hosts with qemu willy nilly with zero
change seen be the guest.
So perhaps the width of pci_addr_t should be a per-device property,
not a host property?
> Honestly, I am extremely sceptical that there would be any
> measurable performance difference.
You may be right, but surely the way to find out is to have a way to
build either, and then compare them. Not have it dictated by the build system.
64-bit ops on 32-bit hosts, especially x86 due to register pressure,
are noticably more expensive than 32-bit ops. The question is whether
they are sparse enough among all the other logic that it doesn't matter.
With a bit of make cleverness it should be quite easy to support a mix
of build-once files and build-few-times files according to the minimal
compile time variations those files depend on - and express those
dependencies is a simple, one-liner way. GNU Make is good for that
sort of thing.
- Jamie
Re: [Qemu-devel] Re: Compile files only once: some planning, Jamie Lokier, 2010/03/24
[Qemu-devel] Re: Compile files only once: some planning, Paolo Bonzini, 2010/03/24
Re: [Qemu-devel] Re: Compile files only once: some planning, Richard Henderson, 2010/03/24
Re: [Qemu-devel] Re: Compile files only once: some planning, Richard Henderson, 2010/03/24
Re: [Qemu-devel] Re: Compile files only once: some planning, Blue Swirl, 2010/03/24