Date: Sun, 14 Jan 2018 08:06:19 -0800 From: Nathan Whitehorn <nwhitehorn@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r327950 - in head/sys/powerpc: aim include powerpc ps3 Message-ID: <ede06fc6-7c34-100c-8a7a-6346cd8cd363@freebsd.org> In-Reply-To: <20180114083036.GX1684@kib.kiev.ua> References: <201801132314.w0DNEra5002692@repo.freebsd.org> <20180113232441.GV1684@kib.kiev.ua> <010d0153-8931-a3c2-db21-dfcbaf848fc0@freebsd.org> <f33e9b1a-28bd-e6cf-4bdb-ec0097c0787d@freebsd.org> <20180114083036.GX1684@kib.kiev.ua>
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On 01/14/18 00:30, Konstantin Belousov wrote: > On Sat, Jan 13, 2018 at 08:31:40PM -0800, Nathan Whitehorn wrote: >> >> On 01/13/18 15:28, Nathan Whitehorn wrote: >>> >>> On 01/13/18 15:24, Konstantin Belousov wrote: >>>> On Sat, Jan 13, 2018 at 11:14:53PM +0000, Nathan Whitehorn wrote: >>>>> +/* >>>>> + * We (usually) have a direct map of all physical memory. All >>>>> + * uses of this macro must be gated by a check on hw_direct_map! >>>>> + * The location of the direct map may not be 1:1 in future, so use >>>>> + * of the macro is recommended; it may also grow an assert that >>>>> hw_direct_map >>>>> + * is set. >>>>> + */ >>>>> +#define PHYS_TO_DMAP(x) x >>>>> +#define DMAP_TO_PHYS(x) x >>>> Take a look at the sys/vm/vm_page.c:vm_page_free_prep() function. >>>> >>> I think the checks in there should work as designed, unless I'm >>> missing something. Am I? >>> -Nathan >>> >> Actually, wait, this is broken if hw_direct_map is not set. I can do an >> #ifdef __powerpc__ hack, but do you have opinions for a better MI flag >> for "yes, the macro is defined but, no, the direct map may not be >> available"? > Exactly. You explicitly noted the need to check for the hw_direct_map > in the comment, so I did not see a need to explain further. > > We intended that PHYS_TO_DMAP/DMAP_TO_PHYS become MI KPI. If the symbols > are present, their semantic is the unconditional presence and usability of > the direct map. > > sf bufs were rototiled with things like SFBUF_OPTIONAL_DIRECT_MAP, but I > dislike it and hope that PHYS_TO_DMAP would be not damaged this way. > That's unfortunate. Is there any reason you need this to be certain at compile time? That seems to be quite restrictive and not to add a huge amount of performance. Given the exciting variety of MMU modes on PowerPC, there is not any way to guarantee the presence of a direct map at compile time, so the alternative is to invent a whole new kernel signalling mechanism for "direct map is almost certainly available, but might not be", which seems strictly worse, or to have a standard API that PowerPC can't use, which also seems worse. -Nathan
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