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Re: [Xen-devel] [PATCH v10 09/20] x86/VPMU: Add public xenpmu.h

On 09/11/2014 02:39 AM, Jan Beulich wrote:
On 10.09.14 at 19:23, <boris.ostrovsky@xxxxxxxxxx> wrote:
On 09/10/2014 10:45 AM, Jan Beulich wrote:
On 04.09.14 at 05:41, <boris.ostrovsky@xxxxxxxxxx> wrote:
+struct xen_pmu_arch {
+    union {
+        struct cpu_user_regs regs;
+        uint8_t pad[256];
+    } r;
Can you remind me again what you need the union and padding for
This structure is laid out in a shared page with a (possibly 32-bit)
guest who need to access fields that follow this union.
Hmm, okay. But how would such a guest make reasonable use of
the regs field then?

When hypervisor is preparing this data for 32-bit consumer in vpmu_do_interrupts() it translates registers to 32-bit version:

        struct compat_cpu_user_regs *cmp;
        gregs = guest_cpu_user_regs();
        cmp = (void *)&vpmu->xenpmu_data->pmu.r.regs;
        XLAT_cpu_user_regs(cmp, gregs);

I remember struggling trying to figure a better way of presenting this but ended up with the (void *) cast. IIRC I tried putting compat_cpu_user_regs into the union but something didn't quite work (with compilation).

And then - why 256 and not 200? struct
cpu_user_regs can't change size anyway. Plus, finally, why do
you expose the GPRs but not any of the other register state?

I wanted to leave some padding in case we decide to add non-GPR registers and keep major version of the interface unchanged (only minor version will bumped). TBH though, I can't think of any non-GPR registers to be ever useful.


I realize I should have thought of these earlier, but better now
than after the new ABI got established.


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