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Re: [Xen-devel] [PATCH 3/4] xen/arm: set vpidr on the pcpu where the vcpu will run

On 16/02/2018 20:31, Stefano Stabellini wrote:
On Fri, 16 Feb 2018, Julien Grall wrote:
Hi Stefano,

On 15/02/18 23:16, Stefano Stabellini wrote:
On big.LITTLE systems not all cores have the same midr. Instead of
initializing the vpidr to the boot cpu midr, set it to the value of the
midr of the pcpu where the vcpu will run.

This way, assuming that the vcpu has been created with the right pcpu
affinity, the guest will be able to read the right vpidr value, matching
the one of the physical cpu.

Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
   xen/arch/arm/domain.c | 19 ++++++++++++++++---
   1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
index 532e824..280125f 100644
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -315,6 +315,22 @@ static void schedule_tail(struct vcpu *prev)
   static void continue_new_vcpu(struct vcpu *prev)
       current->arch.actlr = READ_SYSREG32(ACTLR_EL1);
+    /*
+     * Default the virtual ID to match the physical.
+     *
+     * In case the big.LITTLE systems, a guest should be created with
+     * cpu affinity set so that all vcpus run on the same kind of pcpus.
+     * Warn if it is not the case.

continue_new_vcpu is only called once at domain creation. So this looks
pointless to check that here and probably in ctxt_switch_to.

But I don't want to see such check at every context switch. This is expensive
and we should not impact all the platforms for the benefits of an unsafe

If you really want to do that, then it should only be done when the vCPU is
migrating. That will reduce a lot the performance impact.

I don't want a check for every context switch either. I added it here
because continue_new_vcpu is only called once per vcpu at domain
creation -- it is a one time check. vcpus are supposed to be pinned (or
cpu affinity specified) anyway, so I thought I wouldn't add the check in
vcpu_migrate too. In any case, I am also happy to remove the check
completely, as we have already warned the user enough.
If you agree that continue_new_vcpu is only called once per vCPU. Then I am not sure to understand the purpose of the check. What are you trying to warn the user with that?


Julien Grall

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