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Re: [Xen-devel] [PATCH 1/3] x86/xen: Avoid fast syscall path for Xen PV guests

On Wed, Nov 18, 2015 at 3:21 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Nov 18, 2015 at 12:06 PM, Boris Ostrovsky
> <boris.ostrovsky@xxxxxxxxxx> wrote:
>> After 32-bit syscall rewrite, and specifically after commit 5f310f739b4c
>> ("x86/entry/32: Re-implement SYSENTER using the new C path"), the stack
>> frame that is passed to xen_sysexit is no longer a "standard" one (i.e.
>> it's not pt_regs).
>> Since we end up calling xen_iret from xen_sysexit we don't need to fix
>> up the stack and instead follow entry_SYSENTER_32's IRET path directly
>> to xen_iret.
>> We can do the same thing for compat mode even though stack does not need
>> to be fixed. This will allow us to drop usergs_sysret32 paravirt op (in
>> the subsequent patch)
> Looks generally quite nice.  Minor comments below:
>> --- a/arch/x86/entry/entry_32.S
>> +++ b/arch/x86/entry/entry_32.S
>> @@ -308,7 +308,8 @@ sysenter_past_esp:
>>         movl    %esp, %eax
>>         call    do_fast_syscall_32
>> -       testl   %eax, %eax
>> +       /* XEN PV guests always use IRET path */
>> +       ALTERNATIVE "testl %eax, %eax", "xor %eax, %eax", X86_FEATURE_XENPV
>>         jz      .Lsyscall_32_done
> Could we make this a little less subtle:
> ALTERNATIVE "testl %eax, %eax; lz .Lsyscall_32_done", "jmp
> .Lsyscasll_32_done", X86_FEATURE_XENPV
> Borislav, what do you think?
> Ditto for the others.

Can you just add !xen_pv_domain() to the opportunistic SYSRET check
instead?  Bury the alternatives in that macro, ie.
static_cpu_has(X86_FEATURE_XENPV).  That would likely benefit other
code as well.

Brian Gerst

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