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Re: [Xen-devel] [PATCH v3 07/32] xen/x86: fix arch_set_info_guest for HVM guests

El 23/07/15 a les 18.19, Jan Beulich ha escrit:
>>>> On 23.07.15 at 18:15, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 23/07/15 17:00, Ian Campbell wrote:
>>> On Thu, 2015-07-23 at 17:48 +0200, Roger Pau Monnà wrote:
>>>> El 23/07/15 a les 17.32, Jan Beulich ha escrit:
>>>>>>>> On 23.07.15 at 17:10, <roger.pau@xxxxxxxxxx> wrote:
>>>>>> IMHO introducing a new structure that gets rid of all the PV-only 
>>>>>> fields seems like a good option:
>>>>>> struct vcpu_hvm_context {
>>>>>> #define _VGCF_online                   5
>>>>>> #define VGCF_online                    (1<<_VGCF_online)
>>>>>>     uint32_t flags;                         /* VGCF_* flags      
>>>>>> */
>>>>>>     struct cpu_hvm_user_regs user_regs;     /* User-level CPU 
>>>>>> registers     
>>>>>> */
>>>>>>     /* NB. User pagetable on x86/64 is placed in ctrlreg[1]. */
>>>>>>     uint32_t ctrlreg[8];                    /* CR0-CR7 (control 
>>>>>> registers)  
>>>>>> */
>>>>>>     uint32_t debugreg[8];                   /* DB0-DB7 (debug 
>>>>>> registers)    
>>>>>> */
>>>>>> };
>>>>>> I'm also seriously considering getting rid of ctrlreg and 
>>>>>> debugreg.
>>>>>> Since HVM VCPUs will always be started in 32bit flat mode, it 
>>>>>> doesn't
>>>>>> make sense IMHO to have both the 32 and the 64 version of the 
>>>>>> registers, so cpu_hvm_user_regs is always going to be:
>>>>>> struct cpu_hvm_user_regs {
>>>>>>     uint32_t ebx;
>>>>>>     uint32_t ecx;
>>>>>>     uint32_t edx;
>>>>>>     uint32_t esi;
>>>>>>     uint32_t edi;
>>>>>>     uint32_t ebp;
>>>>>>     uint32_t eax;
>>>>>>     uint32_t eip;
>>>>>>     uint32_t esp;
>>>>>>     uint32_t eflags;
>>>>>>     uint16_t cs;
>>>>>>     uint16_t ss;
>>>>>>     uint16_t es;
>>>>>>     uint16_t ds;
>>>>>>     uint16_t fs;
>>>>>>     uint16_t gs;
>>>>>> };
>>>>>> We could however do something similar to what's done in ARM and 
>>>>>> have
>>>>>> a union of both the 32 and the 64bit registers in case we want to
>>>>>> start the vCPU in 64bit mode sometime in the future.
>>>>> What you gave above is suitable only for VCPUOP_initialise afaict.
>>>>> Did you intend this to be the case?
>>>> Certainly not, this should also be used by XEN_DOMCTL_setvcpucontext.
>>>> I'm afraid I'm missing something obvious, but I don't see why this
>>>> couldn't be used by XEN_DOMCTL_setvcpucontext TBH, can you please 
>>>> clarify?
>>> set/getvcpucontext need to pickle all state for save/restore/migration,
>>> not just the start of day state.
>> HVM migration doesn't use set/getvcpucontext.
>> I would expect an HVMLite-based-PVH to follow suit and just use
>> set/gethvmcontext.
> Oh, right, but that doesn't mean the respective domctl-s should
> produce unconsumable data, or have to guess how to interpret
> what gets passed in. I.e. at the very minimum the domctl-s then
> should be disallowed for PVH if they can#t be made work right.

Right, the problem is that if I introduce this new vcpu_context
structure plain HVM guests are also going to use it.

This is not a problem with the VCPU_initialize op, because it was not
available to HVM guests. But it's going to be a problem for
XEN_DOMCTL_{get/set}vcpucontext consumers, since the structure
taken/returned by the hypercall is going to change when the target is a
HVM guest, and this is a can of worms I might prefer to avoid opening.

One partial solution would be to allow the usage of this new struct only
for the VCPU_initialize hypercall for HVM guests, and keep using the old
structures for the domctls, aiming at removing them once PV support is
dropped from Xen, but that looks half-baked IMHO.


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