hvm trapping mechanism

hvm trapping mechanism

[xennn]

Hi all,

I would like to ask about hvm xen implementation. As far i understood the
trapping-emulating flow is that: 

1. when hvm virtual machine is started vmcs is setup and a vm_exit handler
is registered at: 

 __vmwrite(HOST_RIP, (unsigned long)vmx_asm_vmexit_handler);

2. the hypercall page is defined by hypercall api and that hypercall page
contains the vmexit calls.
3. when guest os issues syscall an hypercall is perfomed by the hypercall
page and vmexit operation is peformed
4. the vmx_vmexit_handler(struct cpu_user_regs *regs) (vmx.c) is invoked to
emulate the hypercall. 

I would like to ask is this flow is right? Is there somethingimportant
missing? 
I would like to ask how the  vmx_vmexit_handler is invoked - may  be on
hardware level or there is some other party at the flow?

best regerds



--
View this message in context: http://xen.1045712.n5.nabble.com/hvm-trapping-mechanism-tp5720529.html
Sent from the Xen - Dev mailing list archive at Nabble.com.

Re: hvm trapping mechanism

[Andrew Cooper]

On 23/12/2013 21:01, xennn wrote:
> Hi all,
>
> I would like to ask about hvm xen implementation. As far i understood the
> trapping-emulating flow is that: 
>
> 1. when hvm virtual machine is started vmcs is setup and a vm_exit handler
> is registered at: 
>
>  __vmwrite(HOST_RIP, (unsigned long)vmx_asm_vmexit_handler);
>
> 2. the hypercall page is defined by hypercall api and that hypercall page
> contains the vmexit calls.
> 3. when guest os issues syscall an hypercall is perfomed by the hypercall
> page and vmexit operation is peformed
> 4. the vmx_vmexit_handler(struct cpu_user_regs *regs) (vmx.c) is invoked to
> emulate the hypercall. 
>
> I would like to ask is this flow is right? Is there somethingimportant
> missing? 
> I would like to ask how the  vmx_vmexit_handler is invoked - may  be on
> hardware level or there is some other party at the flow?
>
> best regerds

Technically, the hypercall page contains 'vmcall' instructions.  There
is no such thing as a vmexit instruction.  A vmexit is the action of the
processor moving from non-root mode into root mode because of an action
requiring the intervention of the hypervisor.

vmx_asm_vmexit_handler is the handler for all vmexits, the vast majority
of which are not from hypercalls.  It includes may other things, such as
validity checks when the guest writes to control registers, or simply
that the real processor received an interrupt and that Xen should handle it.

~Andrew

Re: hvm trapping mechanism

[Liu, Jinsong]

Andrew Cooper wrote:
> On 23/12/2013 21:01, xennn wrote:
>> Hi all,
>> 
>> I would like to ask about hvm xen implementation. As far i
>> understood the trapping-emulating flow is that:
>> 
>> 1. when hvm virtual machine is started vmcs is setup and a vm_exit
>> handler is registered at: 
>> 
>>  __vmwrite(HOST_RIP, (unsigned long)vmx_asm_vmexit_handler);

Right. From os/vmm programmer point of view, vmexit is a h/w process: when guest os running at non-root mode and when some condition satisfied (say, external interrupt, sensitive instructions excuted) it will drop to root mode hypervisor, enter point of which is defined by HOST_RIP of vmcs --> vmx_asm_vmexit_handler --> vmx_vmexit_handler.

You can refer Intel SDM 3, chapter 25 for causes of vmexit, and chapter 27 for detail process of vmexit.

Thanks,
Jinsong

>> 
>> 2. the hypercall page is defined by hypercall api and that hypercall
>> page contains the vmexit calls. 
>> 3. when guest os issues syscall an hypercall is perfomed by the
>> hypercall page and vmexit operation is peformed
>> 4. the vmx_vmexit_handler(struct cpu_user_regs *regs) (vmx.c) is
>> invoked to emulate the hypercall. 
>> 
>> I would like to ask is this flow is right? Is there
>> somethingimportant missing? I would like to ask how the 
>> vmx_vmexit_handler is invoked - may  be on hardware level or there
>> is some other party at the flow? 
>> 
>> best regerds
> 
> Technically, the hypercall page contains 'vmcall' instructions.  There
> is no such thing as a vmexit instruction.  A vmexit is the action of
> the processor moving from non-root mode into root mode because of an
> action requiring the intervention of the hypervisor.
> 
> vmx_asm_vmexit_handler is the handler for all vmexits, the vast
> majority 
> of which are not from hypercalls.  It includes may other things, such
> as validity checks when the guest writes to control registers, or
> simply 
> that the real processor received an interrupt and that Xen should
> handle it. 
> 
> ~Andrew
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel

Re: hvm trapping mechanism

[xennn]

is that correct flow ? 

10x



--
View this message in context: http://xen.1045712.n5.nabble.com/hvm-trapping-mechanism-tp5720529p5720533.html
Sent from the Xen - Dev mailing list archive at Nabble.com.