Re: [PATCH 00/11] arm64/firmware: Software Delegated Exception Interface

[Christoffer Dall <cdall@linaro.org>]

On Wed, Jun 07, 2017 at 10:45:32AM +0100, James Morse wrote:
> Hi Christoffer,
> 
> On 06/06/17 20:59, Christoffer Dall wrote:
> > On Mon, May 15, 2017 at 06:43:48PM +0100, James Morse wrote:
> >> Running the event handler behind VHE-KVM's back has some side effects: The
> >> event handler will blindly use any registers that are shared between the host
> >> and guest. The two that I think matter are TPIDR_EL1, and the debug state. The
> >> guest may have set MDSCR_EL1 so debug exceptions must remain masked. The
> >> guest's TPIDR_EL1 will be used by the event handler if it accesses per-cpu
> >> variables. This needs fixing. The first part of this series juggles KVMs use
> >> of TPIDR_EL2 so that we share it with the host on VHE systems. An equivalent
> >> change for 32bit is on my todo list. (one alternative to this is to have a
> >> parody world switch in the SDEI event handler, but this would mean special
> >> casing interrupted guests, and be an ABI link to KVM.)
> >>
> >> Causing a synchronous exception from an event handler will cause KVM to
> >> hyp-panic, but may silently succeed if the event didn't interrupt a guest.
> >> (I may WARN_ON() if this happens in a later patch). You because of this you
> > 
> > The last sentence here doesn't make much sense to me.
> 
> If this interrupts a VHE-guest, KVM's hyp-vectors remain in vbar_el2. If we then
> take a synchronous exception, KVM will assume this happened during world switch
> and panic.
> In contrast if you didn't interrupt a guest, the kernel vectors are in vbar_el2,
> so the fault will be handled.

Sorry, I actually just meant the grammar... "You because of this you..."

> 
> This 'silently succeed' only occurred to me when writing the cover-letter,
> (hence its probably worded badly). I plan to catch it by save-restoring the
> exception registers, and warning if they've changed as this would hyp-panic if
> you interrupted a guest.
> 

But I didn't understand that before, and didn't realize that 'silently
succeed' may not be what you want.

Thanks for the explanation.

-Christoffer