Re: [Xen-devel] [PATCH RFC v2 1/4] x86/mm: Shadow and p2m changes for PV mem_access

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/08/14 08:33, Jan Beulich wrote:
>>>> On 22.08.14 at 22:02, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 22/08/14 20:48, Aravindh Puthiyaparambil (aravindp) wrote:
>>> Sigh, if only I could bound the CR0.WP solution :-(
>> I wonder whether, in the case that mem_access is enabled, it would be
>> reasonable to perform the CR0.WP sections with interrupts disabled?
> Which still wouldn't cover NMIs (albeit we might be able to live with
> that).

NMIs and MCEs are short, possibly raise softirqs, or call panic().  We
have much larger problems in general if the lack of CR0.WP would
adversely affect the NMI or MCE paths.

> But what's worse - taking faults with interrupts disabled
> requires extra care, and annotating code normally run with
> interrupts enabled with the special .ex_table.pre annotations
> doesn't seem like a very nice route, as that could easily hide other
> problems in the future.

Does it?  In exception_with_ints_disabled, if the .ex_table.pre search
fails, we jump back into the regular handler after the sti label and
continue with the regular handler.

This requires no more careful handling than existing constructs such as
wrmsr_safe() inside a spinlock_irq{,save}() region.

~Andrew

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