[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [V10 PATCH 23/23] PVH xen: introduce vmexit handler for PVH

On 15/08/13 17:37, Tim Deegan wrote:
At 16:51 +0100 on 15 Aug (1376585469), George Dunlap wrote:
On Wed, Aug 7, 2013 at 10:54 AM, Tim Deegan <tim@xxxxxxx> wrote:
At 17:37 -0700 on 06 Aug (1375810631), Mukesh Rathor wrote:
On Fri, 26 Jul 2013 11:45:19 +0100
Tim Deegan <tim@xxxxxxx> wrote:

At 19:30 -0700 on 25 Jul (1374780657), Mukesh Rathor wrote:
On Thu, 25 Jul 2013 17:28:40 +0100
Tim Deegan <tim@xxxxxxx> wrote:

At 18:59 -0700 on 23 Jul (1374605971), Mukesh Rathor wrote:
+/* Just like HVM, PVH should be using "cpuid" from the kernel
mode. */ +static int vmxit_invalid_op(struct cpu_user_regs
*regs) +{
+    if ( guest_kernel_mode(current, regs)
|| !emulate_forced_invalid_op(regs) )
+        hvm_inject_hw_exception(TRAP_invalid_op,
Was this discussed before?  It seems harsh to stop kernel-mode
code from using the pv cpuid operation if it wants to.  In
particular, what about loadable kernel modules?
Yes, few times on the xen mailing list. The only PVH guest, linux
as of now, the pv ops got rewired to use native cpuid, which is
how hvm does it.
Yes, but presumably you want to make it easy for other PV guests to
port to PVH too?
True, but how would not allowing kernel mode emulation impede that?
I fail to understand why a new kernel would wanna use xen signature
emulation over just plain cpuid instruction?
I'm talking about existing PV kernel code that already uses PV CPUID.
And in particular what if that kernel code is in a device driver, or
even a third-party loadable module?  Porting the core kernel from PV to
PVH shouldn't break that code if it doesn't have to.

But TBH my objection is really more aesthetic than anything else.
Restricting the PV CPUID instruction here adds another ragged edge in
the ABI that all kernel writers have to think about, and for little or
no benfit.  And, to be clear, I object to it and this patch does not
have my Ack.
Are you in particular saying that you think PVH guests should be
allowed to use the PV CPUID as a prerequisite for the patch series to
go in?

Correct me if I'm wrong, but:
1. CPUID is the only forced_emulated op at the moment
2. The kernel is the only caller we can reasonably expect to use the
forced_emulated ops
No - the forced-invalid-op CPUID is used from userspace too, e.g. in
tools/misc/xen-detect.c.  The current patch keeps support for userspace
but not for kernel users.  I would prefer to keep it for both, on the
grounds that even in the kernel there may be users of it that the person
porting the kernel does not control (e.g. in third-party modules).

Ah, right. That makes sense then -- we'd want to have the same toolstack binaries for PV and PVH. But then if we're going to have it, I agree that there's no reason not to just go ahead and make it available in all modes. We can just deprecate it in kernel mode.


Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.