Messages in this thread | | | Subject | Re: [PATCH] KVM: x86: VMX: Make smaller physical guest address space support user-configurable | From | Paolo Bonzini <> | Date | Thu, 3 Sep 2020 22:02:34 +0200 |
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On 03/09/20 20:32, Jim Mattson wrote: >> [Checking writes to CR3] would be way too slow. Even the current >> trapping of present #PF can introduce some slowdown depending on the >> workload. > > Yes, I was concerned about that...which is why I would not want to > enable pedantic mode. But if you're going to be pedantic, why go > halfway?
Because I am not sure about any guest, even KVM, caring about setting bits 51:46 in CR3.
>>> Does the typical guest care about whether or not setting any of the >>> bits 51:46 in a PFN results in a fault? >> >> At least KVM with shadow pages does, which is a bit niche but it shows >> that you cannot really rely on no one doing it. As you guessed, the >> main usage of the feature is for machines with 5-level page tables where >> there are no reserved bits; emulating smaller MAXPHYADDR allows >> migrating VMs from 4-level page-table hosts. >> >> Enabling per-VM would not be particularly useful IMO because if you want >> to disable this code you can just set host MAXPHYADDR = guest >> MAXPHYADDR, which should be the common case unless you want to do that >> kind of Skylake to Icelake (or similar) migration. > > I expect that it will be quite common to run 46-bit wide legacy VMs on > Ice Lake hardware, as Ice Lake machines start showing up in > heterogeneous data centers.
If you'll be okay with running _all_ 46-bit wide legacy VMs without MAXPHYADDR emulation, that's what this patch is for. If you'll be okay with running _only_ 46-bit wide VMs without emulation, you still don't need special enabling per-VM beyond the automatic one based on CPUID[0x8000_0008]. Do you think you'll need to enable it for some special 46-bit VMs?
Paolo
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