Re: [PATCH for-10.1] target/arm: Reinstate bogus AArch32 DBGDTRTX register for migration compat

[Fabiano Rosas <farosas@suse.de>]

Peter Maydell <peter.maydell@linaro.org> writes:

> In commit 655659a74a we fixed some bugs in the encoding of the
> Debug Communications Channel registers, including that we were
> incorrectly exposing an AArch32 register at p14, 3, c0, c5, 0.
>
> Unfortunately removing a register is a break of forwards migration
> compatibility for TCG, because we will fail the migration if the
> source QEMU passes us a cpreg which the destination QEMU does not
> have.  We don't have a mechanism for saying "it's OK to ignore this
> sysreg in the inbound data", so for the 10.1 release reinstate the
> incorrect AArch32 register.
>
> (We probably have had other cases in the past of breaking migration
> compatibility like this, but we didn't notice because we didn't test
> and in any case not that many people care about TCG migration
> compatibility.  KVM migration compat is not affected because for KVM
> we treat the kernel as the source of truth for what system registers
> are present.)
>
> Fixes: 655659a74a36b ("target/arm: Correct encoding of Debug Communications Channel registers")
> Reported-by: Fabiano Rosas <farosas@suse.de>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Reviewed-by: Fabiano Rosas <farosas@suse.de>