Date: Fri, 13 Mar 2015 10:47:02 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Nathan Whitehorn <nwhitehorn@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r279937 - in head/sys/powerpc: include powerpc Message-ID: <20150313084702.GV2379@kib.kiev.ua> In-Reply-To: <5502094D.5090001@freebsd.org> References: <201503122115.t2CLFdmi026986@svn.freebsd.org> <20150312212234.GS2379@kib.kiev.ua> <55020547.7050102@freebsd.org> <20150312213530.GT2379@kib.kiev.ua> <5502094D.5090001@freebsd.org>
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On Thu, Mar 12, 2015 at 02:46:53PM -0700, Nathan Whitehorn wrote: > > On 03/12/15 14:35, Konstantin Belousov wrote: > > On Thu, Mar 12, 2015 at 02:29:43PM -0700, Nathan Whitehorn wrote: > >> On 03/12/15 14:22, Konstantin Belousov wrote: > >>> On Thu, Mar 12, 2015 at 09:15:39PM +0000, Nathan Whitehorn wrote: > >>>> Author: nwhitehorn > >>>> Date: Thu Mar 12 21:15:38 2015 > >>>> New Revision: 279937 > >>>> URL: https://svnweb.freebsd.org/changeset/base/279937 > >>>> > >>>> Log: > >>>> Provide VSX context in ucontext(3) API. > >>>> > >>>> Modified: > >>>> head/sys/powerpc/include/ucontext.h > >>>> head/sys/powerpc/powerpc/exec_machdep.c > >>>> > >>>> Modified: head/sys/powerpc/include/ucontext.h > >>>> ============================================================================== > >>>> --- head/sys/powerpc/include/ucontext.h Thu Mar 12 20:14:48 2015 (r279936) > >>>> +++ head/sys/powerpc/include/ucontext.h Thu Mar 12 21:15:38 2015 (r279937) > >>>> @@ -46,6 +46,7 @@ typedef struct __mcontext { > >>>> uint32_t mc_av[2]; > >>>> register_t mc_frame[42]; > >>>> uint64_t mc_fpreg[33]; > >>>> + uint64_t mc_vsxfpreg[32]; /* low-order half of VSR0-31 */ > >>>> } mcontext_t __aligned(16); > >>>> > >>>> #if defined(_KERNEL) && defined(__powerpc64__) > >>>> @@ -60,6 +61,7 @@ typedef struct __mcontext32 { > >>>> uint32_t mc_av[2]; > >>>> uint32_t mc_frame[42]; > >>>> uint64_t mc_fpreg[33]; > >>>> + uint64_t mc_vsxfpreg[32]; /* low-order half of VSR0-31 */ > >>>> } mcontext32_t __aligned(16); > >>>> #endif > >>> It looks as if you broken the ABI compatibility by the change. Am I wrong ? > >>> > >> That is correct. It's a tier-2 platform and -CURRENT, so I'm not sure > >> it's worth the compatibility shims. I'm happy to add them if you think > >> otherwise. > > You are main maintainer of PowerPC port, IMO, so it is your decision. > > > > Note that 'this is current' argument is not applicable, since the change > > also breaks stable/* binaries. > > > > I do understand the argument of PowerPC being tier 2 architecture, but this > > makes me sad. Anyway, it is yours. For x86, I have to introduce > > getcontextx(3) mechanism. > > > This is a good point. I'll try to fix it. Is my understanding of how > this works correct? > > 1. Provide a sysarch() for the extended FPU state. > 2. Implement getcontextx() in the C library to fill extra properties if > required. > 3. Store state for signal trampoline in variable-sized stack area 4. Implement __getcontextx_size() and __fillcontextx2() for use in the deferred signal delivery while libthr is in critical section, see lib/libthr/thread/thr_sig.c:check_deferred_signal(). > > Implementation of (2) seems to rely on having spare members in ucontext, > which PowerPC unfortunately does not have. Is there a way around that? Indeed, this is very unfortunate. My concern is that typical application allocating ucontext_t on stack or by mallocing it, would get silent memory corruption after the extension of mcontext_t. It seems indeed that ABI breakage cannot be completely avoided, but it could be significantly reduced IMO. Is it true that mc_avec is only valid when _MC_AV_VALID bit is set ? If yes, we can introduce another mcontext flag, say _MC_XSTATE_VALID, which is mutually exclusive with the _MC_AC_VALID. The new flag indicates that there is external data, and the data is pointed to by some word placed in the previous mc_avec file, say mc_avec[0]. The altivec registers file content is moved into that external data area as well. Providing the external area size in mc_avec[1] allows to extend that block in the future-compatible manner. This way, applications which use ucontext_t and which are not aware about VSX, get the expected behaviour, possibly without seeing altivec. I.e. in typical case, we do not get random memory corruption. Meantime, I have a question. I looked at the powerpc/include/ucontex.h and tried to match it with the PowerISA specs 2.06 and 2.07. From what I understand, mc_fpreg corresponds to the floating-point registers file, mc_avec and mc_av to the 'Vector Facility Registers'. But I fail to see what would match the + uint64_t mc_vsxfpreg[32]; /* low-order half of VSR0-31 */ file. The 7.2.1 Vector-Scalar Registers says 'Sixty-four 128-bit VSRs are provided'.
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