Hi Yang,
Yes, I have. In particular the FPU state is the following:
FPU registers (XSave):
fcw: 0x0
fsw: 0x0 (top: 0, conditions: , exceptions: )
ftwx: 0xff
last_opcode: 0x0
last_ip: 0x0
last_dp: 0x0
mxcsr_mask: 0x0
mxcsr: 0x1f80
FP Stack:
ST0/0: 0x00000000000000000000 (0)
ST1/1: 0x00000000000000000000 (0)
ST2/2: 0x00000000000000000000 (0)
ST3/3: 0x00000000000000000000 (0)
ST4/4: 0x00000000000000000000 (0)
ST5/5: 0x00000000000000000000 (0)
ST6/6: 0x00000000000000000000 (0)
ST7/7: 0x00000000000000000000 (0)
XMM registers:
0: 0x00000000000000400000000000000000
1: 0x00000000000000000000000000000000
2: 0x00000000000000000000000000000000
3: 0x00000000000000000000000000000000
4: 0x00000000000000000000000000000000
5: 0x00000000000000000000000000000000
6: 0x00000000000000000000000000000000
7: 0x00000000000000000000000000000000
8: 0x00000000000000000000000000000000
9: 0x00000000000000000000000000000000
10: 0x00000000000000000000000000000000
11: 0x00000000000000000000000000000000
12: 0x00000000000000000000000000000000
13: 0x00000000000000000000000000000000
14: 0x00000000000000000000000000000000
15: 0x00000000000000000000000000000000
Do you have particular registers you are wondering about?
Thank you,
Alex
On Sun, Aug 7, 2016 at 7:26 PM, Yang Zhang <yang.zhang.wz@xxxxxxxxx> wrote:
> On 2016/8/5 4:40, Alexandru Duţu wrote:
>>
>> Hi everyone,
>>
>> I am using KVM to run a virtual machine (x86) without having a guest
>> OS. So the machine state is set for long mode before calling KVM_RUN
>> and I am trapping out of KVM on every syscall to emulate the syscall
>> in the program that uses KVM. Also I am trapping out of KVM on
>> pagefaults to deal with stack growth.
>>
>> This has been working great for integer workloads, however I have
>> encoutered some workloads which are using floating point instructions,
>> for example cvtsi2sd to cast an integer to a double, and their result
>> is incorrect.
>>
>> Recently I have discovered that KVM does FPU context save on-demand,
>> meaning when the virtual machine actually uses the FPU [1]. I am
>
>
> Latest KVM will use eagerfpu if host is using eagerfpu.
>
>> wondering if this machanism has some baked in assumptions about
>> running a virtual machine with a guest OS and if a guest OS is not
>> present the FPU state might be altered? Also I have not seen any #NM
>> exceptions, these usually trap out of KVM.
>
>
> #NM exceptions will trap into KVM only when current hardware FPU state not
> belongs to the running VCPU, i.e. first access to FPU.
>
>>
>> The FPU state that I am using [2] seems valid and it does not change
>> when trapping out of KVM for syscalls and pagefaults. In addition, I
>
>
> Have you check the state before vm entry?
>
>> have tried running with multiple linux versions 2.6.29, 3.13 and 4.0.4
>> and all of them give incorrect floating point instructions result,
>> however for different instructions.
>>
>> Thank you,
>>
>
>
> --
> Yang
> Alibaba Cloud Computing
--
Alex
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