linux-kernel - [GIT PULL] KVM: x86: PV clock changes for 6.15

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Message-ID: <20250318180303.283401-4-seanjc@google.com>
Date: Tue, 18 Mar 2025 11:02:58 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Sean Christopherson <seanjc@...gle.com>
Subject: [GIT PULL] KVM: x86: PV clock changes for 6.15

Fix a variety of bugs, flaws, and warts related to KVM's handling of PV clocks
and the associated PVCLOCK_GUEST_STOPPED flag.  Note, there are still a pile of
issues with KVM's PV clock code; hopefully the next version of those changes[*]
comes along sooner than later.

[*] https://lore.kernel.org/all/20240522001817.619072-1-dwmw2@infradead.org

The following changes since commit a64dcfb451e254085a7daee5fe51bf22959d52d3:

  Linux 6.14-rc2 (2025-02-09 12:45:03 -0800)

are available in the Git repository at:

  https://github.com/kvm-x86/linux.git tags/kvm-x86-pvclock-6.15

for you to fetch changes up to 1b3c38050b5cc07f6873f244f845fb6c8549ce85:

  KVM: x86: Override TSC_STABLE flag for Xen PV clocks in kvm_guest_time_update() (2025-02-12 10:45:55 -0800)

----------------------------------------------------------------
KVM PV clock changes for 6.15:

 - Don't take kvm->lock when iterating over vCPUs in the suspend notifier to
   fix a largely theoretical deadlock.

 - Use the vCPU's actual Xen PV clock information when starting the Xen timer,
   as the cached state in arch.hv_clock can be stale/bogus.

 - Fix a bug where KVM could bleed PVCLOCK_GUEST_STOPPED across different
   PV clocks.

 - Restrict PVCLOCK_GUEST_STOPPED to kvmclock, as KVM's suspend notifier only
   accounts for kvmclock, and there's no evidence that the flag is actually
   supported by Xen guests.

 - Clean up the per-vCPU "cache" of its reference pvclock, and instead only
   track the vCPU's TSC scaling (multipler+shift) metadata (which is moderately
   expensive to compute, and rarely changes for modern setups).

----------------------------------------------------------------
Sean Christopherson (11):
      KVM: x86: Don't take kvm->lock when iterating over vCPUs in suspend notifier
      KVM: x86: Eliminate "handling" of impossible errors during SUSPEND
      KVM: x86: Drop local pvclock_flags variable in kvm_guest_time_update()
      KVM: x86: Process "guest stopped request" once per guest time update
      KVM: x86/xen: Use guest's copy of pvclock when starting timer
      KVM: x86: Don't bleed PVCLOCK_GUEST_STOPPED across PV clocks
      KVM: x86: Set PVCLOCK_GUEST_STOPPED only for kvmclock, not for Xen PV clock
      KVM: x86: Pass reference pvclock as a param to kvm_setup_guest_pvclock()
      KVM: x86: Remove per-vCPU "cache" of its reference pvclock
      KVM: x86: Setup Hyper-V TSC page before Xen PV clocks (during clock update)
      KVM: x86: Override TSC_STABLE flag for Xen PV clocks in kvm_guest_time_update()

 arch/x86/include/asm/kvm_host.h |   3 +-
 arch/x86/kvm/x86.c              | 115 ++++++++++++++++++++--------------------
 arch/x86/kvm/xen.c              |  69 +++++++++++++++++++++---
 3 files changed, 121 insertions(+), 66 deletions(-)