From: Shivansh Dhiman <shivansh.dhiman@amd.com>
To: Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>
Cc: <kvm@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
Yosry Ahmed <yosry@kernel.org>,
Shivansh Dhiman <shivansh.dhiman@amd.com>,
"Nikunj A Dadhania" <nikunj@amd.com>,
Ravi Bangoria <ravi.bangoria@amd.com>,
"Tom Lendacky" <thomas.lendacky@amd.com>
Subject: Re: [PATCH v6 01/12] KVM: x86: Preserve DR6.BLD (Bus Lock Detect) when delivering #DB payload
Date: Thu, 1 Oct 2026 05:16:36 +0530 [thread overview]
Message-ID: <52f72fb5-d472-4800-856e-1157be67487d@amd.com> (raw)
In-Reply-To: <20260930193634.3415089-2-seanjc@google.com>
On 01-10-26 01:06, Sean Christopherson wrote:
> Preserve Bus Lock Detect if it isn't present in the payload. Unlike RTM,
> which is explicitly modified on every #DB, DR6.BLD is modified if and only
> if the #DB is due to a (coincident) Bus Lock Detect.
>
> Per Intel's SDM:
>
> Other debug exceptions do not modify this bit.
>
> And AMD's APM
>
> All other #DB exceptions leave DR6[BLD] unmodified,
>
> Restore the explicit RTM handling that was clobbered by commit
> 9a3ecd5e2aa1 ("KVM: X86: Rename DR6_INIT to DR6_ACTIVE_LOW"), as the
> semantics of any given bit are feature specific, i.e. are obviously not
> consistent for active-high versus active-low. In other words, while the
> bug was technically introduced when support for Bus Lock Detect was added,
> it's really that attempt to genericize handling of active-low bits that set
> KVM up to fail.
>
> Fixes: e8ea85fb280e ("KVM: X86: Add support for the emulation of DR6_BUS_LOCK bit")
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested on Turin with Bus Lock Detect enabled in both L1 and L2 guests. The
expected bus lock warnings show up in dmesg. Also, all the KUT tests pass
with Sean's KUT patches applied.
Tested-by: Shivansh Dhiman <shivansh.dhiman@amd.com>
next prev parent reply other threads:[~2026-09-30 23:46 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-30 19:36 [PATCH v6 00/12] KVM: x86: Bus Lock Detect fixes and SVM support Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 01/12] KVM: x86: Preserve DR6.BLD (Bus Lock Detect) when delivering #DB payload Sean Christopherson
2026-09-30 23:46 ` Shivansh Dhiman [this message]
2026-09-30 19:36 ` [PATCH v6 02/12] KVM: x86: Rename kvm_dr6_fixed() => kvm_get_dr6_fixed_1() Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 03/12] KVM: Return an "unsigned long", not "u64" for the fixed-1 DR6 bits Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 04/12] KVM: x86: Force fixed-1 bits in DR6 after synchronizing with hardware Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 05/12] KVM: x86: Set guest's fixed-1 DR6 bits when delivering #DB payload Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 06/12] KVM: x86: Kill off DR6_FIXED_1 to prevent future misuse Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 07/12] KVM: SVM: Add helper to query if LBR virtualization needs to be enabled Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 08/12] KVM: nSVM: Disable LBRV in nested control cache when unsupported Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 09/12] KVM: nSVM: Open code check on LBR virtualization being enabled in vmcb12 Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 10/12] KVM: nSVM: Don't assume all active-low bits DR6 are fixed-1 Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 11/12] KVM: SVM: Add a vCPU-aware helper to get supported DEBUGCTL bits Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 12/12] KVM: SVM: Add support for virtualizating Bus Lock Detect Sean Christopherson
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