Re: [Qemu-devel] [PATCH] q35/mch: implement extended TSEG sizes

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/06/2017 18:10, Laszlo Ersek wrote:
> When the guest writes value 0xffff to this register, the value that can be
> read back is that of "mch.extended-tseg-mbytes" -- unless it remains
> 0xffff. The guest is required to write 0xffff first (as opposed to a
> read-only register) because PCI config space is generally not cleared on
> QEMU reset, and after S3 resume or reboot, new guest firmware running on
> old QEMU could read a guest OS-injected value from this register.

I guess that's also a reason not to make it readonly (that is, it would
require some firmware code anyway to test for "readonlyness" and
distinguish old machine types from new)?

Thanks,

Paolo