Re: [Xen-devel] [PATCH v7 05/18] tools/libxc: support to resume uncooperative HVM guests

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Jan 29, 2016 at 01:27:21PM +0800, Wen Congyang wrote:
> Before this patch:
> 1. suspend
> a. PVHVM and PV: we use the same way to suspend the guest (send the suspend
>    request to the guest). If the guest doesn't support evtchn, the xenstore
>    variant will be used, suspending the guest via XenBus control node.
> b. pure HVM: we call xc_domain_shutdown(..., SHUTDOWN_suspend) to suspend
>    the guest
> 
> 2. Resume:
> a. fast path(fast=1)
>    Do not change the guest state. We call libxl__domain_resume(.., 1) which
>    calls xc_domain_resume(..., 1 /* fast=1*/) to resume the guest.
>    PV:       modify the return code to 1, and than call the domctl:
>              XEN_DOMCTL_resumedomain
>    PVHVM:    same with PV
>    pure HVM: do nothing in modify_returncode, and than call the domctl:

"then"

>              XEN_DOMCTL_resumedomain
> b. slow
>    Used when the guest's state have been changed. Will call
>    libxl__domain_resume(..., 0) to resume the guest.
>    PV:       update start info, and reset all secondary CPU states. Than call
>              the domctl: XEN_DOMCTL_resumedomain
>    PVHVM:    can not be resumed. You will get the following error message:
>                  "Cannot resume uncooperative HVM guests"
>    purt HVM: same with PVHVM

"pure"

> 
> After this patch:
> 1. suspend
>    unchanged
> 
> 2. Resume
> a. fast path:
>    unchanged
> b. slow
>    PV:       unchanged
>    PVHVM:    call XEN_DOMCTL_resumedomain to resume the guest. Because we
>              don't modify the return code, the PV driver will disconnect
>              and reconnect.
>              The guest ends up doing the XENMAPSPACE_shared_info
>              XENMEM_add_to_physmap hypercall and resetting all of its CPU
>              states to point to the shared_info(well except the ones past 32).
>              That is the Linux kernel does that - regardless whether the
>              SCHEDOP_shutdown:SHUTDOWN_suspend returns 1 or not.
>    Pure HVM: call XEN_DOMCTL_resumedomain to resume the guest.

In summary, this patch only changes slow path resume. Further more, it
only affects PVHVM and pure HVM variants.

With you patch, pure HVM is able to resume with effectively the same
path via XEN_DOMCTL_resumedomain, albeit it is done in two functions
(_cooperative and _any).

And according to the recently change in documentation, slow path is
always safe.

I think the commit message can be simplified a bit. This is assuming
using XEN_DOMCTL_resumedomain to resume (PV)HVM in slow path is safe.

===

Use XEN_DOMCTL_resumedomain to resume (PV)HVM guest in slow path

Previously it was not possible to resume PVHVM or pure HVM guest in slow
path because libxc didn't support that.

Using XEN_DOMCTL_resumedomain without modifying guest state  to resume a
guest is considered to be always safe.  Introduce a function to do that
for (PV)HVM guests in slow path resume.

This patch fixes a bug that denies (PV)HVM slow path resume.  This will
enable COLO to work properly:  COLO requires HVM guest to start in the
new context that has been set up by COLO, hence slow path resume is
required.

===

Does this sound right? Especially the wording about safety.

Ian and Ian, you seemed to have suggested Congyang to write the above
commit message. What do you think about my updated one?

> 
> Under COLO, we will update the guest's state(modify memory, cpu's registers,
> device status...). In this case, we cannot use the fast path to resume it.
> Keep the return code 0, and use a slow path to resume the guest. While
> resuming HVM using slow path is not supported currently, this patch is to
> make the resume call to not fail.
> 
> Signed-off-by: Wen Congyang <wency@xxxxxxxxxxxxxx>
> Signed-off-by: Yang Hongyang <hongyang.yang@xxxxxxxxxxxx>
> ---
>  tools/libxc/xc_resume.c | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/libxc/xc_resume.c b/tools/libxc/xc_resume.c
> index 87d4324..4a9b035 100644
> --- a/tools/libxc/xc_resume.c
> +++ b/tools/libxc/xc_resume.c
> @@ -108,6 +108,26 @@ static int xc_domain_resume_cooperative(xc_interface 
> *xch, uint32_t domid)
>      return do_domctl(xch, &domctl);
>  }
>  
> +static int xc_domain_resume_hvm(xc_interface *xch, uint32_t domid)
> +{
> +    DECLARE_DOMCTL;
> +
> +    /*
> +     * The domctl XEN_DOMCTL_resumedomain unpause each vcpu. After
> +     * the domctl, the guest will run.
> +     *
> +     * If it is PVHVM, the guest called the hypercall
> +     *    SCHEDOP_shutdown:SHUTDOWN_suspend
> +     * to suspend itself. We don't modify the return code, so the PV driver
> +     * will disconnect and reconnect.
> +     *
> +     * If it is a HVM, the guest will continue running.
> +     */
> +    domctl.cmd = XEN_DOMCTL_resumedomain;
> +    domctl.domain = domid;
> +    return do_domctl(xch, &domctl);
> +}
> +
>  static int xc_domain_resume_any(xc_interface *xch, uint32_t domid)
>  {
>      DECLARE_DOMCTL;
> @@ -137,10 +157,7 @@ static int xc_domain_resume_any(xc_interface *xch, 
> uint32_t domid)
>       */
>  #if defined(__i386__) || defined(__x86_64__)
>      if ( info.hvm )
> -    {
> -        ERROR("Cannot resume uncooperative HVM guests");
> -        return rc;
> -    }
> +        return xc_domain_resume_hvm(xch, domid);
>  
>      if ( xc_domain_get_guest_width(xch, domid, &dinfo->guest_width) != 0 )
>      {
> -- 
> 2.5.0
> 
> 
> 
> 
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