[Xen-devel] Re: One question to IST stack for PV guest
On 12/18/2009 01:05 AM, Jiang, Yunhong wrote:
Jeremy/Keir, I'm trying to add vMCA injection to pv_ops dom0. Because
currently we didn't have virtual IST stack support, so I plan to use the kernel
stack for vMCE. But Andi told me that this method should have issue if MCE is
injected before syscall handler switches to kernel stack. After checking the
code, seems this apply in pv_ops dom0, since undo_xen_syscall will switch to
user space stack firstly (see following code).
What are the requirements here? Are these events delivered to dom0 to
indicate that something needs attention on the machine, or are they
delivered synchronously to whatever domain is currently running to say
that something bad needs immediate attention?
I'm not sure if we really need to switch to user space stack, or we can simply
place user stack to oldrsp and don't switch the stack at all, since xen
hypervisor has use the kernel stack already.
Another option is to add vIST stack, but that requires changes for dom0/xen
interface and is a bit complex.
What about making the call a bit like the failsafe callback, which
always uses the kernel stack, to deliver these exceptions? That could
reshape the kernel stack to conform to the normal stack frame and then
call the usual arch/x86 handlers.
I checked the 2.6.18 kernel and seems it have no such issue, because syscall
entry in arch/x86_64/kernel/entry-xen.S will use kernel stack directly. (But
vMCE injection may have issue still because it use zeroentry).
BTW, Jeremy, seems vNMI support is not included in pvops dom0, will it be
supported in future?
There's been no call for it so far, so I hadn't worried about it much.
I was thinking it might be useful as a debug tool, but I don't know what
it gets used for normally.
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