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Message-ID: <abcfa1a3-982b-ccf5-5e6f-16e63ac03dbc@linux.alibaba.com>
Date: Tue, 8 Jun 2021 22:01:24 +0800
From: Lai Jiangshan <laijs@...ux.alibaba.com>
To: Sean Christopherson <seanjc@...gle.com>,
Maxim Levitsky <mlevitsk@...hat.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>,
linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
kvm@...r.kernel.org
Subject: Re: [PATCH V2] KVM: X86: fix tlb_flush_guest()
On 2021/6/8 08:03, Sean Christopherson wrote:
> On Tue, Jun 08, 2021, Maxim Levitsky wrote:
>> So this patch *does* fix the windows boot without TDP!
>
> Woot!
>
>> Tested-by: Maxim Levitsky <mlevitsk@...hat.com>
>> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
>
> Lai,
>
> I have a reworded version of your patch sitting in a branch that leverages this
> path to fix similar bugs and do additional cleanup. Any objection to me gathering
> Maxim's tags and posting the version below? I'm more than happy to hold off if
> you'd prefer to send your own version, but I don't want to send my own series
> without this fix as doing so would introduce bugs.
Sean
Wow, thank you, it is an excellent rewording and I'm happy with it.
I'm looking forward to your series and I will remake the "need_sync"[1] patch
to more precisely keep&synchronize roots in few days if there is no other
optimization.
Thanks
Lai
[1]: https://lore.kernel.org/lkml/20210525213920.3340-1-jiangshanlai@gmail.com/
>
> Thanks!
>
> Author: Lai Jiangshan <laijs@...ux.alibaba.com>
> Date: Tue Jun 1 01:22:56 2021 +0800
>
> KVM: x86: Unload MMU on guest TLB flush if TDP disabled to force MMU sync
>
> When using shadow paging, unload the guest MMU when emulating a guest TLB
> flush to all roots are synchronized. From the guest's perspective,
> flushing the TLB ensures any and all modifications to its PTEs will be
> recognized by the CPU.
>
> Note, unloading the MMU is overkill, but is done to mirror KVM's existing
> handling of INVPCID(all) and ensure the bug is squashed. Future cleanup
> can be done to more precisely synchronize roots when servicing a guest
> TLB flush.
>
> If TDP is enabled, synchronizing the MMU is unnecessary even if nested
> TDP is in play, as a "legacy" TLB flush from L1 does not invalidate L1's
> TDP mappgins. For EPT, an explicit INVEPT is required to invalidate
> guest-physical mappings. For NPT, guest mappings are always tagged with
> an ASID and thus can only be invalidated via the VMCB's ASID control.
>
> This bug has existed since the introduction of KVM_VCPU_FLUSH_TLB, but
> was only recently exposed after Linux guests stopped flushing the local
> CPU's TLB prior to flushing remote TLBs (see commit 4ce94eabac16,
> "x86/mm/tlb: Flush remote and local TLBs concurrently").
>
> Tested-by: Maxim Levitsky <mlevitsk@...hat.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> Fixes: f38a7b75267f ("KVM: X86: support paravirtualized help for TLB shootdowns")
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> [sean: massaged comment and changelog]
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1cd6d4685932..3b02528d5ee8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3072,6 +3072,18 @@ static void kvm_vcpu_flush_tlb_all(struct kvm_vcpu *vcpu)
> static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu)
> {
> ++vcpu->stat.tlb_flush;
> +
> + if (!tdp_enabled) {
> + /*
> + * Unload the entire MMU to force a sync of the shadow page
> + * tables. A TLB flush on behalf of the guest is equivalent
> + * to INVPCID(all), toggling CR4.PGE, etc... Note, loading the
> + * MMU will also do an actual TLB flush.
> + */
> + kvm_mmu_unload(vcpu);
> + return;
> + }
> +
> static_call(kvm_x86_tlb_flush_guest)(vcpu);
> }
>
>
>> More notes from the testing I just did:
>>
>> 1. On AMD with npt=0, the windows VM boots very slowly, and then in the task manager
>> I see that it booted with 1 CPU, although I configured it for 3-28 vCPUs (doesn't matter how many)
>> I tested this with several win10 VMs, same pattern repeats.
>
> That's very odd. Maybe it's so slow that the guest gives up on the AP and marks
> it as dead? That seems unlikely though, I can't imagine waking APs would be
> _that_ slow.
>
>> 2. The windows nag screen about "we beg you to open a microsoft account" makes the VM enter a live lock.
>> I see about half million at least VM exits per second due to page faults and it is stuck in 'please wait' screen
>> while with NPT=1 it shows up instantly. The VM has 12 GB of ram so I don't think RAM is an issue.
>>
>> It's likely that those are just result of unoptimized code in regard to TLB flushes,
>> and timeouts in windows.
>> On my Intel laptop, the VM is way faster with EPT=0 and it boots with 3 vCPUs just fine
>> (the laptop has just dual core CPU, so I can't really give more that 3 vCPU to the VM)
>
> Any chance your Intel CPU has PCID? Although the all-contexts INVPCID emulation
> nukes everything, the single-context INVPCID emulation in KVM is optimized to
> (a) sync the current MMU (if necessary) instead of unloading it and (b) free
> only roots with the matching PCID. I believe all other forms of TLB flushing
> that are likely to be used by the guest will lead to KVM unloading the entire
> MMU and rebuilding it from scratch.
>
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