Re: [Xen-devel] [PATCH v6 15/19] x86/VPMU: Add privileged PMU mode

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.05.14 at 04:08, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 05/26/2014 07:48 AM, Jan Beulich wrote:
>>
>>> +                gregs = guest_cpu_user_regs();
>>> +                memcpy(&v->arch.vpmu.xenpmu_data->pmu.r.regs,
>>> +                       gregs, sizeof(struct cpu_user_regs));
>>> +            }
>>> +            else
>>> +                memcpy(&v->arch.vpmu.xenpmu_data->pmu.r.regs,
>>> +                       regs, sizeof(struct cpu_user_regs));
>>>   
>>> -            cmp = (void *)&v->arch.vpmu.xenpmu_data->pmu.r.regs;
>>> -            XLAT_cpu_user_regs(cmp, gregs);
>>> -            memcpy(&v->arch.vpmu.xenpmu_data->pmu.r.regs,
>>> -                   &cmp, sizeof(struct compat_cpu_user_regs));
>>> +            gregs = &v->arch.vpmu.xenpmu_data->pmu.r.regs;
>>> +            gregs->cs = (current->arch.flags & TF_kernel_mode) ? 0 : 0x3;
>> Ah, no - you want to modify the structure here. But you could do this
>> directly on the ->pmu.r.regs field rather than first latching the pointer.
>>
>> And as said before, it doesn't really look correct to simply set ->cs to
>> just the RPL, especially without any comment explaining why this is
>> (a) being done and (b) correct.
> 
> The reason for only passing up the RPL is that's the only field that the 
> guest is interested in (whether the interrupt happened in kernel or user 
> space). I added a comment in the code to this effect.
> 
> Do you think that all fields need to be passed?

How do you know what a guest is interested in? Namely 32-bit OSes
may have uses for more than a single flat code segment, and hence
may have an interest in knowing the full selector.

Jan


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