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Most of the time there's no need to kick the vCPU and deliver the timer event through kvm_xen_inject_timer_irqs(). Use kvm_xen_set_evtchn_fast() directly from the timer callback, and only fall back to the slow path if delivering the timer would block, i.e. if kvm_xen_set_evtchn_fast() returns -EWOULDBLOCK. If delivery fails for any other reason, do nothing and just let it fail silently, as that is what the slow path would end up doing anyways. This gives a significant improvement in timer latency testing (using nanosleep() for various periods and then measuring the actual time elapsed). However, there was a reason[1] the fast path was dropped when this support was first added. The current code holds vcpu->mutex for all operations on the kvm->arch.timer_expires field, and the fast path introduces a potential race condition. Avoid that race by ensuring the hrtimer is (temporarily) cancelled before making changes in kvm_xen_start_timer(), and also when reading the values out for KVM_XEN_VCPU_ATTR_TYPE_TIMER. [1] https://lore.kernel.org/kvm/846caa99-2e42-4443-1070-84e49d2f11d2@redhat.com Signed-off-by: David Woodhouse <dwmw@amazon.co.uk> Reviewed-by: Paul Durrant <paul@xen.org> Link: https://lore.kernel.org/r/f21ee3bd852761e7808240d4ecaec3013c649dc7.camel@infradead.org [sean: massage changelog] Signed-off-by: Sean Christopherson <seanjc@google.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.