Re: [PATCH] xen: vcpu_info reinit error after 'xl save -c' & 'xl restore' on PVOPS VM which has multi-cpu

From: Boris Ostrovsky
Date: Tue Apr 28 2015 - 17:33:19 EST

On 04/28/2015 08:30 AM, Ouyang Zhaowei (Charles) wrote:

On 2015.4.26 7:31, Boris Ostrovsky wrote:
On 04/24/2015 05:30 AM, Ouyang Zhaowei (Charles) wrote:
If a PVOPS VM has multi-cpu the vcpu_info of cpu0 is the member of the structure HYPERVISOR_shared_info,
and the others is not, but after 'xl save -c/restore' the vcpu_info will be reinitialized,
the vcpu_info of all the vcpus will be considered as the member of HYPERVISOR_shared_info.
This will cause the cpu1 and other cpu keep receiving interrupts, and the cpu0 is waiting them to
finish the job.
So we do not reinit the vcpu_info when PVOPS vm is doing 'xl save -c/restore'.

Signed-off-by: Charles Ouyang <ouyangzhaowei@xxxxxxxxxx>
arch/x86/xen/suspend.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/suspend.c b/arch/x86/xen/suspend.c
index d949769..b2bed45 100644
--- a/arch/x86/xen/suspend.c
+++ b/arch/x86/xen/suspend.c
@@ -32,7 +32,8 @@ static void xen_hvm_post_suspend(int suspend_cancelled)
int cpu;
- xen_hvm_init_shared_info();
+ if (!suspend_cancelled)
+ xen_hvm_init_shared_info();
if (xen_feature(XENFEAT_hvm_safe_pvclock)) {
Do we need to call other routines if suspend is canceled?

Also, if suspend is canceled then we don't do xen_irq_resume() if that's what you meant by "vcpu_info will be reinitialized". Were you referring some other re-initialization?

Hi Boris,

Sorry I didn't make myself clear.

About the "vcpu_info reinitialize", I mean in the function "xen_hvm_init_shared_info()" the pointer "xen_vcpu" will be reset and all
point to HYPERVISOR_shared_info->vcpu_info[cpu].

void __ref xen_hvm_init_shared_info(void)
1702 * When xen_hvm_init_shared_info is run at boot time only vcpu 0 is
1703 * online but xen_hvm_init_shared_info is run at resume time too and
1704 * in that case multiple vcpus might be online. */
1705 for_each_online_cpu(cpu) {
1706 /* Leave it to be NULL. */
1707 if (cpu >= MAX_VIRT_CPUS)
1708 continue;
1709 per_cpu(xen_vcpu, cpu) = &HYPERVISOR_shared_info->vcpu_info[cpu];
1710 }
1711 }

But on Xen boot the init function "xen_start_kernel" only set the cpu0 to point to HYPERVISOR_shared_info->vcpu_info[0]

asmlinkage __visible void __init xen_start_kernel(void)

We are talking about HVM guests here and xen_start_kernel is only called for PV. But even if it was, xen_vcpu pointers for other VCPUs are set in xen_vcpu_setup(), which is called when non-boot VCPUs are coming up.

And I wonder whether the actual problem is that we don't call xen_vcpu_setup() on canceled suspend (as we don't need to, really) and therefore if we call xen_hvm_init_shared_info() then per_cpu(xen_vcpu,cpu) for *non-boot* cpus is will become wrong.


1563 /* Don't do the full vcpu_info placement stuff until we have a
1564 possible map and a non-dummy shared_info. */
1565 per_cpu(xen_vcpu, 0) = &HYPERVISOR_shared_info->vcpu_info[0];
1567 local_irq_disable();

Other cpus are set to point to "xen_vcpu_info" in function xen_vcpu_setup().

So after xl save -c/restore, the pointer xen_vcpu will be reset in function "xen_hvm_init_shared_info" and point to a wrong place.
This may cause all the cpus cannot handle irqs except cpu0, so IMHO it's not necessary to call xen_hvm_init_shared_info again if
suspend is canceled.

(The patch itself looks like the right thing to do though).



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