Re: [PATCH 0/2] x86/fred: Fix two problems during the FRED initialization

[Xin Li <xin@zytor.com>]

On 6/21/2024 6:12 AM, Hou Wenlong wrote:
> When I reviewed the FRED code and attempted to implement a FRED-like
> event delivery for my PV guest, I encountered two problems which I may
> have misunderstood.

Hi Wenlong,

Thanks for bringing the issues up.

> 
> One issue is that FRED can be disabled in trap_init(), but
> sysvec_install() can be called before trap_init(), thus the system
> interrupt handler is not installed into the IDT if FRED is disabled
> later. Initially, I attempted to parse the cmdline and decide whether to
> enable or disable FRED after parse_early_param(). However, I ultimately
> chose to always install the system handler into the IDT in
> sysvec_install(), which is simple and should be sufficient.

Which module with a system vector gets initialized before trap_init() is
called?

> Another problem is that the page fault handler (exc_page_fault()) is
> installed into the IDT before FRED is enabled. Consequently, if a #PF is
> triggered in this gap, the handler would receive the wrong CR2 from the
> stack if FRED feature is present. To address this, I added a page fault
> entry stub for FRED similar to the debug entry. However, I'm uncertain
> whether this is enough reason to add a new entry. Perhaps a static key
> may suffice to indicate whether FRED setup is completed and the handler
> can use it.

How could a #PF get triggered during that gap?

Initialization time funnies are really unpleasant.

Thanks!
     Xin