Re: [Xen-devel] [PATCH 1/4] xen/PVH: Replace GDT_ENTRY with explicit constant

[Boris Ostrovsky]

On 05/01/2018 03:53 AM, Roger Pau Monnà wrote:
> On Mon, Apr 30, 2018 at 02:07:43PM -0400, Boris Ostrovsky wrote:
> > On 04/30/2018 12:57 PM, Roger Pau Monnà 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@xxxxxxxxxx>
> > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > ---
> > > >   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.