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Re: [PATCH v1 3/3] util/oslib-posix: Support concurrent os_mem_prealloc(

From: David Hildenbrand
Subject: Re: [PATCH v1 3/3] util/oslib-posix: Support concurrent os_mem_prealloc() invocation
Date: Tue, 20 Jul 2021 16:35:59 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

Of course with the madvise impl we're bypassing the SIGBUS
dance entirely. This is good for people with new kernels,
but is this SIGBUS stuff safe for older kernels ?

It remains broken with old kernels I guess. There isn't too much that we can
do: disabling prealloc=on once the VM is running breaks existing use cases.

Ok, while refactoring this, could you add a scary warning next to the
sigaction calls mentioning that this code is not likely to play well
with qemu's other handling of sigbus, as a reminder to future reviewers.

Sure thing!

Fortunately, running into that race seems to be rare, at least I never hear

The failure mode is likely to be silent or easily mis-interpreted

Is there any value in emitting a one-time per process warning message
on stderr if we take the old codepath post-startup ?

Will look into emitting a warning when running this code while the VM is already running. (hope it won't be too ugly to have vm state checks in util/oslib-posix).


David / dhildenb

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