Re: [RFC Part1 PATCH v3 13/17] x86/io: Unroll string I/O when SEV is active

Arnd Bergmann
2017-07-26T10:45:17Z

On Tue, Jul 25, 2017 at 11:51 AM, David Laight <David.Laight@aculab.com> wrote:
> From: Brijesh Singh
>> Sent: 24 July 2017 20:08
>> From: Tom Lendacky <thomas.lendacky@amd.com>
>>
>> Secure Encrypted Virtualization (SEV) does not support string I/O, so
>> unroll the string I/O operation into a loop operating on one element at
>> a time.
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
>> ---
>> arch/x86/include/asm/io.h | 26 ++++++++++++++++++++++----
>> 1 file changed, 22 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
>> index e080a39..2f3c002 100644
>> --- a/arch/x86/include/asm/io.h
>> +++ b/arch/x86/include/asm/io.h
>> @@ -327,14 +327,32 @@ static inline unsigned type in##bwl##_p(int port) \
>> \
>> static inline void outs##bwl(int port, const void *addr, unsigned long count) \
>> {

This will clash with a fix I did to add a "memory" clobber
for the traditional implementation, see
https://patchwork.kernel.org/patch/9854573/

> Is it even worth leaving these as inline functions?
> Given the speed of IO cycles it is unlikely that the cost of calling a real
> function will be significant.
> The code bloat reduction will be significant.

I think the smallest code would be the original "rep insb" etc, which
should be smaller than a function call, unlike the loop. Then again,
there is a rather small number of affected device drivers, almost all
of them for ancient hardware that you won't even build in a 64-bit
x86 kernel, see the list below. The only user I found that is actually
still relevant is drivers/tty/hvc/hvc_xen.c, which uses it for the early
console.

Arnd

---
Full list for reference

$ git grep -wl '\(__ide_\|\)\(in\|out\)s\(b\|w\|l\)' drivers sound/
drivers/atm/horizon.c
drivers/cdrom/gdrom.c
drivers/gpu/drm/vmwgfx/vmwgfx_msg.h
drivers/ide/ide-io-std.c
drivers/isdn/hardware/avm/avmcard.h
drivers/isdn/hardware/eicon/divasmain.c
drivers/isdn/hardware/mISDN/avmfritz.c
drivers/isdn/hardware/mISDN/iohelper.h
drivers/isdn/hardware/mISDN/netjet.c
drivers/isdn/hardware/mISDN/w6692.c
drivers/isdn/hisax/asuscom.c
drivers/isdn/hisax/avm_a1.c
drivers/isdn/hisax/avm_a1p.c
drivers/isdn/hisax/avm_pci.c
drivers/isdn/hisax/diva.c
drivers/isdn/hisax/elsa.c
drivers/isdn/hisax/gazel.c
drivers/isdn/hisax/hisax_fcpcipnp.c
drivers/isdn/hisax/ix1_micro.c
drivers/isdn/hisax/mic.c
drivers/isdn/hisax/netjet.c
drivers/isdn/hisax/niccy.c
drivers/isdn/hisax/saphir.c
drivers/isdn/hisax/sedlbauer.c
drivers/isdn/hisax/sportster.c
drivers/isdn/hisax/teles3.c
drivers/isdn/hisax/w6692.c
drivers/isdn/hisax/w6692.h
drivers/media/rc/winbond-cir.c
drivers/mtd/spi-nor/aspeed-smc.c
drivers/net/appletalk/cops.c
drivers/net/arcnet/arcdevice.h
drivers/net/arcnet/com20020.c
drivers/net/ethernet/3com/3c509.c
drivers/net/ethernet/3com/3c515.c
drivers/net/ethernet/3com/3c574_cs.c
drivers/net/ethernet/3com/3c589_cs.c
drivers/net/ethernet/8390/axnet_cs.c
drivers/net/ethernet/8390/mcf8390.c
drivers/net/ethernet/8390/ne.c
drivers/net/ethernet/8390/ne2k-pci.c
drivers/net/ethernet/8390/pcnet_cs.c
drivers/net/ethernet/8390/smc-ultra.c
drivers/net/ethernet/amd/nmclan_cs.c
drivers/net/ethernet/fujitsu/fmvj18x_cs.c
drivers/net/ethernet/hp/hp100.c
drivers/net/ethernet/smsc/smc9194.c
drivers/net/ethernet/smsc/smc91c92_cs.c
drivers/net/ethernet/smsc/smc91x.h
drivers/net/ethernet/xircom/xirc2ps_cs.c
drivers/net/sb1000.c
drivers/net/wan/sbni.c
drivers/net/wireless/cisco/airo.c
drivers/net/wireless/intersil/hostap/hostap_cs.c
drivers/net/wireless/intersil/hostap/hostap_plx.c
drivers/net/wireless/marvell/libertas/if_cs.c
drivers/net/wireless/wl3501_cs.c
drivers/parport/parport_pc.c
drivers/pcmcia/m32r_cfc.c
drivers/scsi/NCR53c406a.c
drivers/scsi/aha152x.c
drivers/scsi/eata_pio.c
drivers/scsi/fdomain.c
drivers/scsi/g_NCR5380.c
drivers/scsi/imm.c
drivers/scsi/nsp32_io.h
drivers/scsi/pcmcia/nsp_io.h
drivers/scsi/pcmcia/sym53c500_cs.c
drivers/scsi/ppa.c
drivers/scsi/qlogicfas408.c
drivers/scsi/sym53c416.c
drivers/staging/comedi/drivers/adl_pci9111.c
drivers/staging/comedi/drivers/cb_pcidas.c
drivers/staging/comedi/drivers/das16m1.c
drivers/staging/comedi/drivers/das1800.c
drivers/staging/iio/adc/ad7606_par.c
drivers/tty/hvc/hvc_xen.c
drivers/tty/isicom.c
drivers/tty/rocket_int.h
drivers/usb/host/isp1362.h
drivers/usb/musb/blackfin.c
sound/drivers/opl4/opl4_lib.c
sound/isa/gus/gus_dram.c
sound/isa/gus/gus_pcm.c