Messages in this thread | | | Subject | Re: [Xen-devel] [PATCH 1/4] xen/PVH: Replace GDT_ENTRY with explicit constant | From | Boris Ostrovsky <> | Date | Tue, 1 May 2018 08:16:04 -0400 |
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On 05/01/2018 03:53 AM, Roger Pau Monné wrote: > On Mon, Apr 30, 2018 at 02:07:43PM -0400, Boris Ostrovsky wrote: >> On 04/30/2018 12:57 PM, Roger Pau Monné wrote: >>> On Mon, Apr 30, 2018 at 12:23:36PM -0400, Boris Ostrovsky wrote: >>>> Latest binutils release (2.29.1) will no longer allow proper computation >>>> of GDT entries on 32-bits, with warning: >>>> >>>> arch/x86/xen/xen-pvh.S: Assembler messages: >>>> arch/x86/xen/xen-pvh.S:150: Warning: shift count out of range (32 is not between 0 and 31) >>>> arch/x86/xen/xen-pvh.S:150: Warning: shift count out of range (40 is not between 0 and 31) >>>> arch/x86/xen/xen-pvh.S:150: Warning: shift count out of range (32 is not between 0 and 31) >>>> arch/x86/xen/xen-pvh.S:152: Warning: shift count out of range (32 is not between 0 and 31) >>>> arch/x86/xen/xen-pvh.S:152: Warning: shift count out of range (40 is not between 0 and 31) >>>> arch/x86/xen/xen-pvh.S:152: Warning: shift count out of range (32 is not between 0 and 31) >>>> >>>> Use explicit value of the entry instead of using GDT_ENTRY() macro. >>>> >>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com> >>>> Cc: stable@vger.kernel.org >>>> --- >>>> arch/x86/xen/xen-pvh.S | 6 +++--- >>>> 1 file changed, 3 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/arch/x86/xen/xen-pvh.S b/arch/x86/xen/xen-pvh.S >>>> index e1a5fbe..934f7d4 100644 >>>> --- a/arch/x86/xen/xen-pvh.S >>>> +++ b/arch/x86/xen/xen-pvh.S >>>> @@ -145,11 +145,11 @@ gdt_start: >>>> .quad 0x0000000000000000 /* NULL descriptor */ >>>> .quad 0x0000000000000000 /* reserved */ >>>> #ifdef CONFIG_X86_64 >>>> - .quad GDT_ENTRY(0xa09a, 0, 0xfffff) /* __KERNEL_CS */ >>>> + .quad 0x00af9a000000ffff /* __BOOT_CS */ >>>> #else >>>> - .quad GDT_ENTRY(0xc09a, 0, 0xfffff) /* __KERNEL_CS */ >>>> + .quad 0x00cf9a000000ffff /* __BOOT_CS */ >>> Maybe it would be cleaner to use something like: >> >> I actually considered all of these and ended up with a raw number >> because it seems to be a convention in kernel (and Xen too, apparently) >> to use raw values in .S files. >> >> Kernel is using now GDT_ENTRY_INIT() which is a C macro. There is one >> other location where GDT_INIT() is used (arch/x86/boot/pm.c) and, >> incidentally, it also generates this warning IIRC. >> >> I really don't want to move definition to C code just to use a macro --- >> I don't think C code needs to be exposed to this GDT. >> >> >>> >>> .word 0xffff /* limit */ >>> .word 0 /* base */ >>> .byte 0 /* base */ >>> .byte 0x9a /* access */ >>> #ifdef CONFIG_X86_64 >>> .byte 0xaf /* flags plus limit */ >>> #else >>> .byte 0xcf /* flags plus limit */ >>> #endif >>> .byte 0 /* base */ >> >> >> I, in fact, started with something like this. But if you repeat this 4 >> times you will probably see why I decided against it ;-) > > Heh, right. Maybe a .macro to generate those? Or this is all too much > for just a couple of GDT entries anyway...
That's what I thought. Especially given that assembly code seems to be using raw values.
> > For long mode however you could use simpler values, AFAICT the code > segment in long mode could be simplified to: > > 0x00209a0000000000 > > Because the base/limit have no effect.
True. However, we are sharing the DS (and later GS) descriptors between 32- and 64-it modes. I can separate them if you think it makes sense.
-boris
> > In any case I'm not specially inclined either way, and maybe using > similar values for 32 and 64bit modes makes this easier to understand > (and decode if needed). > > Roger. >
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