Re: [Xen-devel] Crash with paravirt-ops 2.6.31.6 kernel

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2009-11-25 at 21:24 +0000, Jeremy Fitzhardinge wrote:
> On 11/25/09 02:22, Jan Beulich wrote:
> > Okay, I think I spotted the relevant difference: 2.6.18 and forward ports
> > set VGCF_in_syscall only when returning from 64-bit system calls (through
> > ret_from_sys_call) - 32-bit syscalls (regardless of the entry path taken)
> > return through int_ret_from_sys_call. 32-bit guest kernels shouldn't be
> > affected by this, as compat mode returns from the hypervisor
> > (compat_restore_all_guest) always use iret.
> >   
> 
> I think dropping the VCGF_in_syscall flag is the simplest possible fix
> then.  There doesn't seem to be a huge benefit to using sysret in this
> case.  Does this look OK?

Looks OK and works for me in practice too.

Ian.

> 
>     J
> 
> Subject: [PATCH] xen: use iret for return from 64b kernel to 32b usermode
> 
> If Xen wants to return to a 32b usermode with sysret it must use the
> right form.  When using VCGF_in_syscall to trigger this, it looks at
> the code segment and does a 32b sysret if it is FLAT_USER_CS32.
> However, this is different from __USER32_CS, so it fails to return
> properly if we use the normal Linux segment.
> 
> So avoid the whole mess by dropping VCGF_in_syscall and simply use
> plain iret to return to usermode.
> 
> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

Tested-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

> 
> diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
> index 02f496a..f681d55 100644
> --- a/arch/x86/xen/xen-asm_64.S
> +++ b/arch/x86/xen/xen-asm_64.S
> @@ -96,7 +96,7 @@ ENTRY(xen_sysret32)
>       pushq $__USER32_CS
>       pushq %rcx
>  
> -     pushq $VGCF_in_syscall
> +     pushq $0
>  1:   jmp hypercall_iret
>  ENDPATCH(xen_sysret32)
>  RELOC(xen_sysret32, 1b+1)
> 
> 



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