From: Paolo Bonzini <pbonzini@redhat.com>
To: Jim Mattson <jmattson@google.com>
Cc: Mohammed Gamal <mgamal@redhat.com>,
kvm list <kvm@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Sean Christopherson <sean.j.christopherson@intel.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH] KVM: x86: VMX: Make smaller physical guest address space support user-configurable
Date: Thu, 3 Sep 2020 22:02:34 +0200 [thread overview]
Message-ID: <208da546-e8e3-ccd5-9686-f260d07b73fd@redhat.com> (raw)
In-Reply-To: <CALMp9eS6O18WcEyw8b6npRSazsyKiGtBjV+coZVGxDNU1JEOsQ@mail.gmail.com>
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
next prev parent reply other threads:[~2020-09-03 20:02 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-03 14:11 [PATCH] KVM: x86: VMX: Make smaller physical guest address space support user-configurable Mohammed Gamal
2020-09-03 17:57 ` Jim Mattson
2020-09-03 18:03 ` Paolo Bonzini
2020-09-03 18:32 ` Jim Mattson
2020-09-03 20:02 ` Paolo Bonzini [this message]
2020-09-03 21:26 ` Jim Mattson
2020-09-23 13:45 ` Paolo Bonzini
[not found] ` <CALMp9eTHbhwfdq4Be=XcUG9z82KK8AapQeVmsdH=mGdQ_Yt2ug@mail.gmail.com>
2020-09-23 15:19 ` Paolo Bonzini
2020-09-28 15:34 ` Qian Cai
2020-09-29 11:59 ` Qian Cai
2020-09-29 12:26 ` Paolo Bonzini
2020-09-29 13:39 ` Qian Cai
2020-09-29 14:47 ` Paolo Bonzini
2020-10-02 17:28 ` Naresh Kamboju
2020-10-02 17:30 ` Paolo Bonzini
2021-01-16 0:08 ` Jim Mattson
2021-01-18 10:18 ` Mohammed Gamal
2021-06-21 18:01 ` Jim Mattson
2021-06-22 12:59 ` Mohammed Gamal
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