svn commit: r270850 - in head/sys: i386/i386 i386/include i386/isa x86/acpica

[Konstantin Belousov]

On Fri, Sep 05, 2014 at 10:44:05AM -0400, John Baldwin wrote:
> On Friday, September 05, 2014 4:43:05 am Konstantin Belousov wrote:
> > On Thu, Sep 04, 2014 at 10:50:25PM -0400, John Baldwin wrote:
> > > On Tuesday, September 02, 2014 06:41:27 PM Konstantin Belousov wrote:
> > > > On Tue, Sep 02, 2014 at 11:00:57AM -0400, John Baldwin wrote:
> > > > > I thought about that.  I could easily make a parallel array, or perhaps
> > > > > use a separate 'susppcb' structure that includes a pcb and the savefpu
> > > > > union and change susppcbs to be an array of those.  Which do you prefer? 
> > > > > If we want to move some state out of the PCB on amd64 into this, then a
> > > > > separate struct for susppcbs might be the sanest.
> > > > 
> > > > Yes, separate structure seems to be a way forward.
> > > 
> > > Please see www.freebsd.org/~jhb/patches/susppcb.patch  Note that I moved
> > > fpususpend() out into a C function on amd64 so that resumectx() could still 
> > > operate on just a pcb.  This also makes savectx and resumectx more symmetric
> > > and matches what I ended up doing on i386.  This is tested for suspend and
> > > resume on both i386 and amd64.
> > 
> > The implementation of fpuresume() in C is definitely an improvement.
> > 
> > You only moved the fpu context to the susppcb, I think this is good for
> > now, we will want to move other bits later.
> > 
> > Do we need to keep pcb layout for KBI compat ?  I remember that pcb
> > size is asserted to properly fit into pcpu area for percpu zones.
> > But why keep the layout ?  I.e. moving all padding bits to the end.
> 
> I wasn't sure.  I thought the padding was there for ABI reasons.  If we don't
> need KBI compat, I would much rather consolidate all the padding at the end.
The padding is due to functional requirements. I do not see KBI
requirements that would cause us to keep the layout, at least in HEAD.

> 
> > There is one weird detail, not touched by your patch.  Amd64 resume
> > path calls initializecpu(), while i386 does not.  I do not see any
> > use for the call, the reload of CRX registers by trampoline/resumectx
> > should already set everything to working state.  E.g., enabling XMM
> > in CR4 after fpu state is restored looks strange.
> 
> I can test that.
> 
> > Overall, it looks fine.  Do you prefer to have alloc_fpusave() on i386 ?
> 
> Well, it might be nice to have XSAVE on i386.  I'm not sure if Intel has
> any 32-bit only chips planned that will use AVX or MPX, etc.  If they are,
> then I do think AVX on i386 would be nice to have.  Barring XSAVE I think
> we can just use a static savefpu on i386 for now.
I mean that having alloc_fpusave() would allow to avoid several #ifdefs
by using pointer to save area on i386 as well.

> 
> We might also consider removing support for 486sx CPUs and requiring an
> on-CPU FPU for i386.  If we do that we might able to use a common fpu.c
> which would be even nicer.

IMO merging fpu.c and npx.c is very non-trivial. First obstacle
is the differences between i386 and amd64 fpu context layouts
(software-imposed).

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