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Re: [Xen-devel] Boot time trap handling

On 12/05/14 13:09, Jan Beulich wrote:
>>>> On 12.05.14 at 12:40, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On the BSP, we:
>> * load the empty 'idt_table'
>> * patch ignore_int() into every entry (and indeed, bottom first which is
>> rather unsafe)
> What is it that you consider "rather unsafe" here?

We load the lower part first with the present bit set, before loading
the top 32 bits of the entry point, into an active IDT.

As we exceedingly unlikely to actually take an interrupt/exception while
doing this, we get away with it.

>> * enter __start_xen()
>> * patch in early_page_fault()
>> * perform large mounts of setup
>> * enter trap_init()
>> * patch the real trap handlers into 'idt_table'
>> * Set up ISTs for #DF, #NMI, #MCE in 'idt_table'
>> * Set up IST stacks in our local TSS
>> * Load our local TSS
>> On APs, we:
>> * Load mmu_cr4_features, including CR4.MCE
>> * Load the BSPs 'idt_table', complete with IST information
>> * Set up IST stacks in our local TSS
>> * Load our local TSS
>> * Switch onto our own local memcpy() of the BSPs idt.
>> In both cases, until we have loaded the TSS, we risk trying to take an
>> MCE or NMI without a TSS loaded.  I cant spot which contributory
>> exception would be generated, but I suspect #NP, or possibly #TS.  With
>> #DF set up in the same way, we will triple fault.
>> Looking at the real trap handlers, they appear to be safe from almost
>> the start of __start_xen().
> That of course may need closer inspection, and documenting the
> results in an eventually evolving patch description.

My meaning here was more "there is nothing which is an obvious problem
to making this work".

I will of course do an analysis of each trap and reason about its early
safety, possibly with patches to tweak the behaviour to ensure safety.

>>  Therefore, I propose:
>> * The boot critical region with an empty idtr gets extended slightly
>> into the top of __start_xen() and start_secondary()
>> * Inside this critical region, set set up and load the TSS.
>> * Load ourselves onto our local idt.
>> * Load cr4, after the MCE entry path is valid.
>> This has the added advantage that we gain full bugframe and extable
>> support for the earlier parts of setup.
>> Is there anything I have overlooked, or does this plan look plausible?
> Looks all reasonable, as long as there's not going to be any window
> without (or with a non-working, or with a simplistic) exception handler
> that would become larger than what it is now.
> Jan

I think that the overall window without exception handling will reduce
substantially, mainly due to the BSP changes.

The AP window isn't changing length much, but is changing from the
current unsafe pseudo handling to no handling until it is all safe to.

One point I forgot to say was that I plan to leave the reserved
exception vectors with non-present descriptors.  This way we get a fatal
GPF indicating the exact vector, rather than a PCPU lockup optionally
followed by an NMI watchdog timeout when another pcpu times out waiting
for the locked up pcpu to rendezvous in the time calibration code.


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