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Re: [Xen-devel] [PATCH RFC] pvh: clearly specify used parameters in vcpu_guest_context

On 16/11/13 01:56, Mukesh Rathor wrote:
On Fri, 15 Nov 2013 17:12:27 -0800
Mukesh Rathor <mukesh.rathor@xxxxxxxxxx> wrote:

On Fri, 15 Nov 2013 15:56:52 -0800
Mukesh Rathor <mukesh.rathor@xxxxxxxxxx> wrote:

On Fri, 15 Nov 2013 13:56:54 -0800
Mukesh Rathor <mukesh.rathor@xxxxxxxxxx> wrote:

On Fri, 15 Nov 2013 16:45:47 +0000
"Jan Beulich" <JBeulich@xxxxxxxx> wrote:

We had talked about this while ago, but upon boot, the first thing
a vcpu needs is access to kernel data structure. (A secondary vcpu
is bootstrapped way up into the kernel). It would be possible to
get rid of gs_base_kernel, but will take some work on the linux
side. I can try and test it out, and let you guys know.
Ok, looking at this more, I can hack cpu_bringup_and_idle() in
linux to include a static variable for cpuid, which is the least a
vcpu needs to know first thing. But, I think that would not work
when vcpu hotplug support is added. Another option would be to pass
cpuid in one of the registers, say rdi. Thus, rdi == cpuid will be
passed to VCPUOP_initialise. In bringup function, the booting vcpu
can then load it's own gs based on the cpuid. If linux folks,
konrad (CCd), is OK with this, we can remove gs_base_kernel.
Otherwise, it's such a small thing, hopefually it can stay.

Here's the changes needed on linux side to remove gs_base_kernel.
Since, I imagine vcpu hotplug would go thru the same path, it would
work there also. If this looks OK to everybody, we can remove
gs_base_kernel from the VCPUOP_initialise call.


diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index aa89bbf..54d1022 100644
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -99,14 +99,8 @@ static void cpu_bringup(void)
        wmb();                  /* make sure everything is out
Rats, just realized this is not going to make sense because you
don't have the earlier patch:
"PVH: simplify cpu_initialize_context. add pvh_set_bringup_context()"

So, just look for it at:

https://oss.oracle.com/git/mrathor/linux.git/ in branch tmp2.

In a nutshell, for PVH we only need to pass eip, rsp, cr3, and
gs_base_kernel. If we pass cpuid in rdi for context setting, we can get
rid of gs_base_kernel.

Since, PVH is now merged, Konrad I'll officially send you the
patches you see in my above tree after the merge to v3.12. I'll do
that next week, lmk what tree the patches should be based on for

The patches were checked in on the understanding that the interface was not yet finalized. I definitely think we should not submit the patches in Linux for inclusion until we have it sorted out. Originally we were thinking of this for 4.5, but we may well be able to get the interface nailed down for 4.4.


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