Date: Fri, 31 Jan 2020 10:13:58 +0100 From: Hans Petter Selasky <hps@selasky.org> To: Konstantin Belousov <kostikbel@gmail.com>, Rick Macklem <rmacklem@uoguelph.ca> Cc: "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: <f8551c4c-0447-4103-76f1-710c4885a2ec@selasky.org> In-Reply-To: <20200130233734.GV4808@kib.kiev.ua> References: <YTBPR01MB3374AA25792499A796DB7CAADD040@YTBPR01MB3374.CANPRD01.PROD.OUTLOOK.COM> <20200130233734.GV4808@kib.kiev.ua>
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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.
--HPS
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