>>> On 08.11.17 at 12:55, <jgross@xxxxxxxx> wrote: > On 08/11/17 12:18, Jan Beulich wrote: >>>>> On 08.11.17 at 10:07, <jgross@xxxxxxxx> wrote: >>> In case we are booted via the default boot entry by a generic loader >>> like grub or OVMF it is necessary to distinguish between a HVM guest >>> with a device model supporting legacy devices and a PVH guest without >>> device model. >>> >>> PVH guests will always have x86_platform.legacy.no_vga set and >>> x86_platform.legacy.rtc cleared, while both won't be true for HVM >>> guests. >>> >>> Test for both conditions in the guest_late_init hook and set xen_pvh >>> to true if they are met. >> >> This sounds pretty fragile to me: I can't see a reason why a proper >> HVM guest couldn't come without VGA and RTC. That's not possible >> today, agreed, but certainly an option down the road if virtualization >> follows bare metal's road towards being legacy free. > > From guest's perspective: what is the difference between a legacy free > HVM domain and PVH? In the end the need for differentiating is to avoid > access to legacy features in PVH as those would require a device model. My point is that "legacy free" would likely be reached over time (and even once fully reached, hybrid configurations would be possible). I.e. there could be a setup with PIC, but with neither VGA nor RTC. That's still not PVH then. Nor do all legacy features require a device model in the first place - some of them are being emulated entirely in the hypervisor. Furthermore, PVH absolutely requires guest awareness afaict, while legacy-free pure HVM guests (with an OS only aware of the possible absence of legacy devices) would still be possible. Jan
