Re: [Qemu-devel] [PATCH v3 0/4] target-arm: Handle tagged addresses when loading PC

[Peter Maydell]

On 13 October 2016 at 20:09, Tom Hanson <address@hidden> wrote:
> Looking at arm_cpu_do_interrupt_aarch64() and the ARM spec, the
> new PC value is always an offset from the appropriate VBAR. The
> only place I can find the the VBAR being set is at boot time
> (i.e. UEFI).

Any guest system software can set the VBAR any time it likes.
In practice it gets set once at bootup and then left that way
because there's no good reason to move it aronud.

> Can the boot code use a tagged pointer to specify the VBAR?

Yes, exactly, you can have a tagged pointer in the VBAR.
The point is that the spec says that when the value is read
out of the VBAR the tag bits must handled appropriately:
check the pseudocode AArch64.TakeException(), which calls
BranchTo(VBAR[] + vect_offset, ...)
and BranchTo() handles the tag bits (in the same way as
any other 'branch to arbitrary new PC value').

thanks
-- PMM