Date: Fri, 31 Jan 2020 14:31:44 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Hans Petter Selasky <hps@selasky.org> Cc: Rick Macklem <rmacklem@uoguelph.ca>, "freebsd-current@FreeBSD.org" <freebsd-current@freebsd.org> Subject: Re: easy way to work around a lack of a direct map on i386 Message-ID: <20200131123144.GW4808@kib.kiev.ua> In-Reply-To: <f8551c4c-0447-4103-76f1-710c4885a2ec@selasky.org> References: <YTBPR01MB3374AA25792499A796DB7CAADD040@YTBPR01MB3374.CANPRD01.PROD.OUTLOOK.COM> <20200130233734.GV4808@kib.kiev.ua> <f8551c4c-0447-4103-76f1-710c4885a2ec@selasky.org>
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On Fri, Jan 31, 2020 at 10:13:58AM +0100, Hans Petter Selasky wrote:
> On 2020-01-31 00:37, Konstantin Belousov wrote:
> > On Thu, Jan 30, 2020 at 11:23:02PM +0000, Rick Macklem wrote:
> > > Hi,
> > >
> > > The current code for KERN_TLS uses PHYS_TO_DMAP()
> > > to access unmapped external pages on m_ext.ext_pgs
> > > mbufs.
> > > I also need to do this to implement RPC-over-TLS.
> > >
> > > The problem is that some arches, like i386, don't
> > > support PHYS_TO_DMAP().
> > >
> > > Since it appears that there will be at most 4 pages on
> > > one of these mbufs, my thinking was...
> > > - Acquire four pages of kva from the kernel_map during
> > > booting.
> > > - Then just use pmap_qenter() to fill in the physical page
> > > mappings for long enough to copy the data.
> > >
> > > Does this sound reasonable?
> > > Is there a better way?
> >
> > Use sfbufs, they should work on all arches. In essence, they provide MI
> > interface to DMAP where possible. I do not remember did I bumped the
> > limit for i386 after 4/4 went in.
> >
> > There is currently no limits for sfbufs use per subsystem, but I think it
> > is not very likely to cause too much troubles. Main rule is to not sleep
> > waiting for more sfbufs if you already own one..
>
> In the DRM-KMS LinuxKPI we have:
>
> void *
> kmap(vm_page_t page)
> {
> #ifdef LINUXKPI_HAVE_DMAP
> vm_offset_t daddr;
>
> daddr = PHYS_TO_DMAP(VM_PAGE_TO_PHYS(page));
>
> return ((void *)daddr);
> #else
> struct sf_buf *sf;
>
> sched_pin();
> sf = sf_buf_alloc(page, SFB_NOWAIT | SFB_CPUPRIVATE);
> if (sf == NULL) {
> sched_unpin();
> return (NULL);
> }
> return ((void *)sf_buf_kva(sf));
> #endif
> }
>
> void
> kunmap(vm_page_t page)
> {
> #ifdef LINUXKPI_HAVE_DMAP
> /* NOP */
> #else
> struct sf_buf *sf;
>
> /* lookup SF buffer in list */
> sf = sf_buf_alloc(page, SFB_NOWAIT | SFB_CPUPRIVATE);
>
> /* double-free */
> sf_buf_free(sf);
> sf_buf_free(sf);
>
> sched_unpin();
> #endif
> }
>
> I think that is the fastest way to do this.
So the kmap address is only valid on the CPU that called the function ?
This is strange, I was not able to find mention of it in references to
kmap.
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