On Wed, Jun 17, 2020 at 06:43:20PM +0200, Laszlo Ersek wrote: > On 06/17/20 18:14, Laszlo Ersek wrote: > > On 06/17/20 15:46, Dr. David Alan Gilbert wrote: > >> * Laszlo Ersek (ler...@redhat.com) wrote: > >>> On 06/16/20 19:14, Guilherme Piccoli wrote: > >>>> Thanks Gerd, Dave and Eduardo for the prompt responses! > >>>> > >>>> So, I understand that when we use "-host-physical-bits", we are > >>>> passing the *real* number for the guest, correct? So, in this case we > >>>> can trust that the guest physbits matches the true host physbits. > >>>> > >>>> What if then we have OVMF relying in the physbits *iff* > >>>> "-host-phys-bits" is used (which is the default in RH and a possible > >>>> machine configuration on libvirt XML in Ubuntu), and we have OVMF > >>>> fallbacks to 36-bit otherwise? > >>> > >>> I've now read the commit message on QEMU commit 258fe08bd341d, and the > >>> complexity is simply stunning. > >>> > >>> Right now, OVMF calculates the guest physical address space size from > >>> various range sizes (such as hotplug memory area end, default or > >>> user-configured PCI64 MMIO aperture), and derives the minimum suitable > >>> guest-phys address width from that address space size. This width is > >>> then exposed to the rest of the firmware with the CPU HOB (hand-off > >>> block), which in turn controls how the GCD (global coherency domain) > >>> memory space map is sized. Etc. > >>> > >>> If QEMU can provide a *reliable* GPA width, in some info channel (CPUID > >>> or even fw_cfg), then the above calculation could be reversed in OVMF. > >>> We could take the width as a given (-> produce the CPU HOB directly), > >>> plus calculate the *remaining* address space between the GPA space size > >>> given by the width, and the end of the memory hotplug area end. If the > >>> "remaining size" were negative, then obviously QEMU would have been > >>> misconfigured, so we'd halt the boot. Otherwise, the remaining area > >>> could be used as PCI64 MMIO aperture (PEI memory footprint of DXE page > >>> tables be darned). > >>> > >>>> Now, regarding the problem "to trust or not" in the guests' physbits, > >>>> I think it's an orthogonal discussion to some extent. It'd be nice to > >>>> have that check, and as Eduardo said, prevent migration in such cases. > >>>> But it's not really preventing OVMF big PCI64 aperture if we only > >>>> increase the aperture _when "-host-physical-bits" is used_. > >>> > >>> I don't know what exactly those flags do, but I doubt they are clearly > >>> visible to OVMF in any particular way. > >> > >> The firmware should trust whatever it reads from the cpuid and thus gets > >> told from qemu; if qemu is doing the wrong thing there then that's our > >> problem and we need to fix it in qemu. > > > > This sounds good in practice, but -- as Gerd too has stated, to my > > understanding -- it has potential to break existing usage. > > > > Consider assigning a single device with a 32G BAR -- right now that's > > supposed to work, without the X-PciMmio64Mb OVMF knob, on even the "most > > basic" hardware (36-bit host phys address width, and EPT supported). If > > OVMF suddenly starts trusting the CPUID from QEMU, and that results in a > > GPA width of 40 bits (i.e. new OVMF is run on old QEMU), then the big > > BAR (and other stuff too) could be allocated from GPA space that EPT is > > actually able to deal with. --> regression for the user. > > s/able/unable/, sigh. :/
I was confused for a while, thanks for the clarification. :) So, I'm trying to write down which additional guarantees we want to give to guests, exactly. I don't want the documentation to reference "host physical address bits", but actual behavior we don't emulate. What does "unable to deal with" means in this specific case? I remember MAXPHYADDR mismatches make EPT treatment of of reserved bits not be what guests would expect from bare metal, but can somebody point out to the specific guest-visible VCPU behavior that would cause a regression in OVMF? Bonus points if anybody can find the exact Intel SDM paragraph we fail to implement. > > > > > Sometimes I can tell users "hey given that you're building OVMF from > > source, or taking it from a 3rd party origin anyway, can you just run > > upstream QEMU too", but most of the time they just want everything to > > continue working on a 3 year old Ubuntu LTS release or whatever. :/ > > Agreed. It wouldn't reasonable to ask guest software to unconditionally trust the data we provide to it after we provided incorrect data to guests for [*checks git log*] 13 years. -- Eduardo
