On 03/11/17 16:10, Boris Ostrovsky wrote: > On 11/03/2017 10:59 AM, Juergen Gross wrote: >> On 03/11/17 15:36, Boris Ostrovsky wrote: >>> On 11/03/2017 10:24 AM, Juergen Gross wrote: >>>> On 03/11/17 15:07, Roger Pau Monné wrote: >>>>> On Fri, Nov 03, 2017 at 01:50:11PM +0100, Juergen Gross wrote: >>>>>> On 03/11/17 13:17, Roger Pau Monné wrote: >>>>>>> On Fri, Nov 03, 2017 at 01:00:46PM +0100, Juergen Gross wrote: >>>>>>>> On 29/09/17 17:51, Roger Pau Monné wrote: >>>>>>>>> On Fri, Sep 29, 2017 at 03:33:58PM +0000, Juergen Gross wrote: >>>>>>>>>> On 29/09/17 17:24, Roger Pau Monné wrote: >>>>>>>>>>> On Fri, Sep 29, 2017 at 02:46:53PM +0000, Juergen Gross wrote: >>>>>>>>>>> Then, I also wonder whether it would make sense for this grub to >>>>>>>>>>> load >>>>>>>>>>> the kernel using the PVH entry point or the native entry point. >>>>>>>>>>> Would >>>>>>>>>>> it be possible to boot a Linux kernel up to the point where cpuid >>>>>>>>>>> can >>>>>>>>>>> be used inside of a PVH container? >>>>>>>>>> I don't think today's Linux allows that. This has been discussed >>>>>>>>>> very thoroughly at the time Boris added PVH V2 support to the kernel. >>>>>>>>> OK, I'm not going to insist on that, but my plans for FreeBSD is to >>>>>>>>> make the native entry point capable of booting inside of a PVH >>>>>>>>> container up to the point where cpuid (or whatever method) can be used >>>>>>>>> to detect the environment. >>>>>>>> Looking more thoroughly into the Linux boot code I think this could >>>>>>>> work for Linux, too. But only if we can tell PVH from HVM in the guest. >>>>>>>> How would you do that in FreeBSD? Via flags in the boot params? This >>>>>>>> would the have to be done in the boot loader (e.g. grub or OVMF). >>>>>>> My plan was not to differentiate between HVM and PVH, but rather to >>>>>>> make use of the ACPI information in order to decide which devices are >>>>>>> available and which are not inside of a PVH guest. >>>>>>> >>>>>>> For example in the FADT "IA-PC Boot Architecture Flags" field for PVH >>>>>>> we already set "VGA Not Present" and "CMOS RTC Not Present". There >>>>>>> might be other flags/fields that must be set, but I would like to >>>>>>> avoid having a CPUID bit or similar saying "PVH", because then Xen >>>>>>> will be tied to always providing the same set of devices in PVH >>>>>>> containers. >>>>>> Why? This would depend on the semantics tied to the flag. It could just >>>>>> mean "don't assume availability of legacy stuff" (e.g. BIOS calls). >>>>>> >>>>>> Linux would have a problem with the ACPI approach as it would try BIOS >>>>>> calls way before it is initializing its ACPI handling. So in Linux I'd >>>>>> need another way to tell I'm running in PVH mode, e.g. a "no legacy" >>>>>> bit in the Xen HVM cpuid leaf. >>>>> If you are booted from the PVH entry point, there's no BIOS or UEFI >>>>> (ie: no firmware), if you are booted from the BIOS entry point there's >>>>> a BIOS and the same applies to UEFI. How does Linux differentiate >>>>> whether it's booted from BIOS or UEFI? >>>> They use different entries. >>> In fact, we had a discussion with Matt Fleming (Linux EFI maintainer) to >>> see if we can use EFI entry point to also be able to boot PVH guest but >>> found some issues with that approach, which is why we ended up with a >>> dedicated PVH entry point. >>> >>> I am curious though, Juergen --- what do we need besides zeropage to >>> allow us to boot PVH from startup_64? >> Oh, you are right. I managed to get lost in the early boot paths. >> >> Only setting up the hyperpage seems to be missing, but this should be >> doable. And setting xen_pvh, of course. > > That last part was actually my question --- do we need to have xen_pvh > set before we get to xen-specific code for the first time (which I think > is init_hypervisor_platform()) from startup_64? > > Because if we do --- who will set it?
This should be easily testable: Just set an internal indicator in xen_prepare_pvh() and copy that to xen_pvh in init_hypervisor_platform() and see whether the kernel still comes up in PVH mode. The same can be done with setting up the hypercall page. Just building a kernel trying that... Juergen _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org https://lists.xen.org/xen-devel
