Re: [Qemu-devel] [PATCH 3/7] target-arm: mark up cpregs for no-migrate or raw access

[Christoffer Dall <christoffer.dall@linaro.org>]

On Thu, May 30, 2013 at 11:27:01PM +0100, Peter Maydell wrote:
> On 30 May 2013 23:13, Christoffer Dall <christoffer.dall@linaro.org> wrote:
> > What happens with registers which don't have the raw_write function set
> > (even though the write function imposes some access checks or has side
> > effects) and also is not marked as ARM_CP_NO_MIGRATE,
> 
> In the general case what happens is that we probably don't
> sync (or migrate, for TCG) the register properly, because
> we'll use the standard write function and get whatever it
> does. (Note that mistakes in annotation don't affect KVM
> migration because we always trust the kernel's register
> list and values and work with them directly; we don't indirect
> through the TCG CPUState structures to migrate the data.)
> 
> The alternative would seem to be to require a raw_read/write
> function to be explicitly specified if there's a read/write
> function (even if it's specified to be the same thing), but
> that seemed to me like it would add a lot of boilerplate for
> most register descriptions. Do you think it would be better
> anyway, or do you have a better idea?

Depends on how many places you add the raw functions.  I was thinking
about whether the absence of such a function could substitute the need
for the NO_MIGRATE flag, but, eh, there's probably other uses for having
that flag so it's not really preferred.

You probably did the best thing.

> 
> > CONTEXTIDR seems to be such an example. ?
> 
> In this specific case I decided it was safe to let the non-raw
> write function do a tlb_flush(). Looking again that is kinda
> expensive though, so we should probably mark these registers
> up with raw_write functions.
> 

Migration is sort of an expensive operation, so not sure if it's worth
it.

I am mostly worries about the case where we would miss raw read/write
functions and that could be hard to track down in the case where
migration fails, but I don't really have great suggestions on how to
ensure this.

-Christoffer