January 19, 2020 6:58 AM, "Claudio Chinicz" wrote:
> Hi,
>
> As far as I understand, HVMs should be faster than PV. With the latter, the
> OS makes hyper calls to
> the hyper-visor while HVMs simply see virtualized hardware through the hyper
> visor.
I always thought the opposite was true. PVs should be faster because they don't
have to go through a hardware emulator, they can just communicate with the
hypervisor using an efficient protocol. I think that was the only reason for
creating PVs in the first place (other than historical reasons before VT-x
maybe). Otherwise they would have made HVM the only mode, much like KVM does.
Btw, Qubes 4.0 uses PVH as the default mode, except for PCI passthru VMs. The
reason is that the PV, while efficient, has become really insecure and is
becoming deprecated. PVH runs the VM under VT-x like an HVM, but also has
special guest-side PV drivers to make I/O faster by bypassing the emulated
hardware. For example, PV(H) guests use the blkback driver (e.g. xvda) while
HVM guests use a virtual SATA controller (e.g. sda) which is emulated by Qemu
in userspace. Similarly, PV(H) uses netfront (e.g. "vif0" network interface)
while HVMs use an emulated ethernet device (e.g. "eth0"). For PCI devices,
PV(H) uses pcifront, however I think this is deprecated, which is why Qubes 4.0
uses HVM for all passthru VMs. HVMs use the platform features (e.g. IOMMU) to
passthru the actual PCI hardware.
HVM:
DomU driver -> Xen -> Qemu -> Dom0 hardware driver -> Dom0 hardware
PV:
DomU PV driver -> Xen -> Dom0 hardware
At least that's my understanding.
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