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Re: [Qemu-devel] KVM and variable-endianness guest CPUs
From: |
Christoffer Dall |
Subject: |
Re: [Qemu-devel] KVM and variable-endianness guest CPUs |
Date: |
Fri, 17 Jan 2014 20:24:01 -0800 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Fri, Jan 17, 2014 at 06:52:57PM +0000, Peter Maydell wrote:
> On 17 January 2014 17:53, Peter Maydell <address@hidden> wrote:
> > Specifically, the KVM API says "here's a uint8_t[] byte
> > array and a length", and the current QEMU code treats that
> > as "this is a byte array written as if the guest CPU
> > (a) were in TARGET_WORDS_BIGENDIAN order and (b) wrote its
> > I/O access to this buffer rather than to the device".
> >
> > The KVM API docs don't actually specify the endianness
> > semantics of the byte array, but I think that that really
> > needs to be nailed down. I can think of a couple of options:
> > * always LE
> > * always BE
> > [these first two are non-starters because they would
> > break either x86 or PPC existing code]
> > * always the endianness the guest is at the time
> > * always some arbitrary endianness based purely on the
> > endianness the KVM implementation used historically
> > * always the endianness of the host QEMU binary
> > * something else?
> >
> > Any preferences? Current QEMU code basically assumes
> > "always the endianness of TARGET_WORDS_BIGENDIAN",
> > which is pretty random.
>
> Having thought a little more about this, my opinion is:
>
> * we should specify that the byte order of the mmio.data
> array is host kernel endianness (ie same endianness
> as the QEMU process itself) [this is what it actually
> is, I think, for all the cases that work today]
I completely agree, given that it's too late to be set on always LE/BE,
I think the natural choice is something that allows a user to cast the
byte array to an appropriate pointer type and dereference it.
And I think we need to amend the KVM API docs to specify this.
--
Christoffer
Re: [Qemu-devel] KVM and variable-endianness guest CPUs, Alexander Graf, 2014/01/20
- Re: [Qemu-devel] KVM and variable-endianness guest CPUs, Christoffer Dall, 2014/01/20
- Re: [Qemu-devel] KVM and variable-endianness guest CPUs, Victor Kamensky, 2014/01/22
- Re: [Qemu-devel] KVM and variable-endianness guest CPUs, Anup Patel, 2014/01/22
- Re: [Qemu-devel] [Qemu-ppc] KVM and variable-endianness guest CPUs, Alexander Graf, 2014/01/22
- Re: [Qemu-devel] [Qemu-ppc] KVM and variable-endianness guest CPUs, Victor Kamensky, 2014/01/22