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From: Greg Kurz <gkurz@linux.vnet.ibm.com>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	qemu-ppc@nongnu.org, qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [Qemu-ppc] [PATCH 5/7] target-ppc: gdbstub: fix altivec registers for little-endian guests
Date: Wed, 20 Jan 2016 08:55:59 +0100	[thread overview]
Message-ID: <20160120085559.668a133c@bahia.huguette.org> (raw)
In-Reply-To: <20160120021357.GH27454@voom.redhat.com>

On Wed, 20 Jan 2016 13:13:57 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Tue, Jan 19, 2016 at 10:59:20AM +0100, Greg Kurz wrote:
> > On Mon, 18 Jan 2016 13:25:19 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > On Fri, Jan 15, 2016 at 04:00:38PM +0100, Greg Kurz wrote:  
> > > > Altivec registers are 128-bit wide. They are stored in memory as two
> > > > 64-bit values that must be byteswapped when the guest is little-endian.
> > > > Let's reuse the ppc_maybe_bswap_register() helper for this.
> > > > 
> > > > We also need to fix the ordering of the 64-bit elements according to
> > > > the target endianness, for both system and user mode.
> > > > 
> > > > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>    
> > > 
> > > What bothers me about this is that avr_need_swap() now depends on both
> > > host and guest endianness.  However the VSCR and VRSAVE swap - like
> > > the swaps for GPRs and FPRs - uses ppc_maybe_bswap_register() which
> > > depends only on guest endianness.
> > > 
> > > Why does altivec depend on the host endianness?
> > >   
> > 
> > This has always been the case:
> > 
> > commit b4f8d821e5211bbb51a278ba0fc4a4db2d581221
> > Author: aurel32 <aurel32@c046a42c-6fe2-441c-8c8c-71466251a162>
> > Date:   Sat Jan 24 15:08:09 2009 +0000
> > 
> >     target-ppc: Add Altivec register read/write using XML
> > 
> > [...]
> > 
> > +static int gdb_get_avr_reg(CPUState *env, uint8_t *mem_buf, int n)
> > +{
> > +    if (n < 32) {
> > +#ifdef WORDS_BIGENDIAN
> > +        stq_p(mem_buf, env->avr[n].u64[0]);
> > +        stq_p(mem_buf+8, env->avr[n].u64[1]);
> > +#else
> > +        stq_p(mem_buf, env->avr[n].u64[1]);
> > +        stq_p(mem_buf+8, env->avr[n].u64[0]);
> > +#endif
> > +        return 16;
> > +    }
> > 
> > My understanding is that gdb expects registers to be presented with
> > the target endianness but QEMU have them in host endianness.
> > 
> > The ppc_maybe_bswap_register() helper is needed to fix 64-bit values
> > according to the target effective endianness because stq_p() always
> > consider both ppc64 and ppc64le to be big endian.
> > 
> > Here, we have a 128-bit register that we break into two 64-bit values
> > in memory. Each quad word has to be fixed by ppc_maybe_bswap_register().
> > But we also have to reorder these quad words if the host endianness
> > differs from the target's one. This is the purpose of avr_need_swap().  
> 
> Ok, understood.  I've merged the series to ppc-for-2.6
> 

Cool ! Thanks.

--
Greg

  reply	other threads:[~2016-01-20  7:56 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-01-15 15:00 [Qemu-devel] [PATCH 0/7] target-ppc: gdbstub: endiannes fixes and VSX support Greg Kurz
2016-01-15 15:00 ` [Qemu-devel] [PATCH 1/7] target-ppc: kvm: fix floating point registers sync on little-endian hosts Greg Kurz
2016-01-18  2:16   ` David Gibson
2016-01-18  8:51     ` Greg Kurz
2016-01-19  0:55       ` David Gibson
2016-01-19 12:10         ` Greg Kurz
2016-01-15 15:00 ` [Qemu-devel] [PATCH 2/7] target-ppc: rename and export maybe_bswap_register() Greg Kurz
2016-01-15 15:00 ` [Qemu-devel] [PATCH 3/7] target-ppc: gdbstub: fix float registers for little-endian guests Greg Kurz
2016-01-15 15:00 ` [Qemu-devel] [PATCH 4/7] target-ppc: gdbstub: introduce avr_need_swap() Greg Kurz
2016-01-15 15:00 ` [Qemu-devel] [PATCH 5/7] target-ppc: gdbstub: fix altivec registers for little-endian guests Greg Kurz
2016-01-18  2:25   ` David Gibson
2016-01-19  9:59     ` Greg Kurz
2016-01-20  2:13       ` [Qemu-devel] [Qemu-ppc] " David Gibson
2016-01-20  7:55         ` Greg Kurz [this message]
2016-01-15 15:00 ` [Qemu-devel] [PATCH 6/7] target-ppc: gdbstub: fix spe " Greg Kurz
2016-01-15 15:00 ` [Qemu-devel] [PATCH 7/7] target-ppc: gdbstub: Add VSX support Greg Kurz

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