On Wed, Dec 12, 2018 at 05:08:39PM +0800, Yu Zhang wrote: > On Tue, Dec 11, 2018 at 05:25:27PM -0200, Eduardo Habkost wrote: > > Some downstream distributions of QEMU set host-phys-bits=on by > > default. This worked very well for most use cases, because > > phys-bits really didn't have huge consequences. The only > > difference was on the CPUID data seen by guests, and on the > > handling of reserved bits. > > > > This changed in KVM commit 855feb673640 ("KVM: MMU: Add 5 level > > EPT & Shadow page table support"). Now choosing a large > > phys-bits value for a VM has bigger impact: it will make KVM use > > 5-level EPT even when it's not really necessary. This means > > using the host phys-bits value may not be the best choice. > > > > Management software could address this problem by manually > > configuring phys-bits depending on the size of the VM and the > > amount of MMIO address space required for hotplug. But this is > > not trivial to implement. > > > > However, there's another workaround that would work for most > > cases: keep using the host phys-bits value, but only if it's > > smaller than 48. This patch makes this possible by introducing a > > new "-cpu" option: "host-phys-bits-limit". Management software > > or users can make sure they will always use 4-level EPT using: > > "host-phys-bits=on,host-phys-bits-limit=48". > > > > This behavior is still not enabled by default because QEMU > > doesn't enable host-phys-bits=on by default. But users, > > management software, or downstream distributions may choose to > > change their defaults using the new option. > > > > Signed-off-by: Eduardo Habkost <ehabk...@redhat.com> > > Thanks, Eduardo. One question is, should we check host-phys-bits-limit > against 48? If not, how about we just say in the commit message, that > the suggested value of host-phys-bits-limit is no bigger than 48 to > ensure a 4-level EPT? :-)
I'm not sure I understood the question. I tried to document this at: | [...] Management software | or users can make sure they will always use 4-level EPT using: | "host-phys-bits=on,host-phys-bits-limit=48". -- Eduardo