Re: [Xen-devel] [PATCH 2/3] X86: MPX support for PV guest

["Liu, Jinsong" <jinsong.liu@xxxxxxxxx>]

Jan Beulich wrote:
>>>> On 11.11.13 at 09:39, "Liu, Jinsong" <jinsong.liu@xxxxxxxxx> wrote:
>> From 56d4b02f595c75b3884b36e54c880ec7019680bc Mon Sep 17 00:00:00
>> 2001 From: Liu Jinsong <jinsong.liu@xxxxxxxxx>
>> Date: Fri, 8 Nov 2013 01:40:59 +0800
>> Subject: [PATCH 2/3] X86: MPX support for PV guest
>> 
>> Signed-off-by: Xudong Hao <xudong.hao@xxxxxxxxx>
>> Acked-by: Liu Jinsong <jinsong.liu@xxxxxxxxx>
>> 
>> ---
>>  tools/libxc/xc_cpuid_x86.c |    3 ++-
>>  xen/arch/x86/traps.c       |    3 ++-
>>  2 files changed, 4 insertions(+), 2 deletions(-)
>> 
>> diff --git a/tools/libxc/xc_cpuid_x86.c b/tools/libxc/xc_cpuid_x86.c
>> index bcb7a59..4c922a8 100644 --- a/tools/libxc/xc_cpuid_x86.c
>> +++ b/tools/libxc/xc_cpuid_x86.c
>> @@ -504,7 +504,8 @@ static void xc_cpuid_pv_policy(
>>                          bitmaskof(X86_FEATURE_BMI2) |
>>                          bitmaskof(X86_FEATURE_ERMS) |
>>                          bitmaskof(X86_FEATURE_RTM)  |
>> -                        bitmaskof(X86_FEATURE_FSGSBASE));
>> +                        bitmaskof(X86_FEATURE_FSGSBASE) |
>> +                        bitmaskof(X86_FEATURE_MPX));          else
>>              regs[1] = 0;
>>          regs[0] = regs[2] = regs[3] = 0;
>> diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
>> index 77c200b..4ecf7d5 100644
>> --- a/xen/arch/x86/traps.c
>> +++ b/xen/arch/x86/traps.c
>> @@ -826,7 +826,8 @@ static void pv_cpuid(struct cpu_user_regs *regs)
>>                    cpufeat_mask(X86_FEATURE_BMI2) |
>>                    cpufeat_mask(X86_FEATURE_ERMS) |
>>                    cpufeat_mask(X86_FEATURE_RTM)  |
>> -                  cpufeat_mask(X86_FEATURE_FSGSBASE));
>> +                  cpufeat_mask(X86_FEATURE_FSGSBASE) |
>> +                  cpufeat_mask(X86_FEATURE_MPX));
>>          else
>>              b = 0;
>>          a = c = d = 0;
> 
> So here you purely mechanically enable the feature. But did you
> consider how it would work? How is the MSR being handled? How
> do you switch between (guest) kernel and user mode? And there's
> no word about what would need to be done when Xen wanted to
> use MPX for its own purposes too.
> 
> In the shape it is now, the above is a clear NAK.
> 
> Jan

[MPX can probably be enabled for HVM guests only anyway] 
Let's do not expose MPX to PV.

Thanks,
Jinsong

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