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| author |   David Woodhouse <dwmw@amazon.co.uk> | 2022-03-03 15:41:12 +0000 |
|---|---|---|
| committer |   Paolo Bonzini <pbonzini@redhat.com> | 2022-04-02 05:34:41 -0400 |
| commit | cf1d88b36ba7e83bdaa50bccc4c47864e8f08cbe (patch) | |
| tree | 1e6264614c9f79e91364fa1dd988ee8a14d4584f /Documentation | |
| parent | KVM: Use enum to track if cached PFN will be used in guest and/or host (diff) | |
| download | linux-dev-cf1d88b36ba7e83bdaa50bccc4c47864e8f08cbe.tar.xz linux-dev-cf1d88b36ba7e83bdaa50bccc4c47864e8f08cbe.zip | |
KVM: Remove dirty handling from gfn_to_pfn_cache completely
It isn't OK to cache the dirty status of a page in internal structures
for an indefinite period of time.
Any time a vCPU exits the run loop to userspace might be its last; the
VMM might do its final check of the dirty log, flush the last remaining
dirty pages to the destination and complete a live migration. If we
have internal 'dirty' state which doesn't get flushed until the vCPU
is finally destroyed on the source after migration is complete, then
we have lost data because that will escape the final copy.
This problem already exists with the use of kvm_vcpu_unmap() to mark
pages dirty in e.g. VMX nesting.
Note that the actual Linux MM already considers the page to be dirty
since we have a writeable mapping of it. This is just about the KVM
dirty logging.
For the nesting-style use cases (KVM_GUEST_USES_PFN) we will need to
track which gfn_to_pfn_caches have been used and explicitly mark the
corresponding pages dirty before returning to userspace. But we would
have needed external tracking of that anyway, rather than walking the
full list of GPCs to find those belonging to this vCPU which are dirty.
So let's rely *solely* on that external tracking, and keep it simple
rather than laying a tempting trap for callers to fall into.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220303154127.202856-3-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/virt/kvm/api.rst | 4 |
1 files changed, 4 insertions, 0 deletions
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index 1ec05e6bfa6f..04b26c2a7159 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -5293,6 +5293,10 @@ type values: KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO Sets the guest physical address of the vcpu_info for a given vCPU. + As with the shared_info page for the VM, the corresponding page may be + dirtied at any time if event channel interrupt delivery is enabled, so + userspace should always assume that the page is dirty without relying + on dirty logging. KVM_XEN_VCPU_ATTR_TYPE_VCPU_TIME_INFO Sets the guest physical address of an additional pvclock structure |