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Anthoine Bourgeois 1706904ef0 xen/netfront: Fix TX response spurious interrupts 
[ Upstream commit 114a2de6fa ]

We found at Vates that there are lot of spurious interrupts when
benchmarking the xen-net PV driver frontend. This issue appeared with a
patch that addresses security issue XSA-391 (b27d47950e "xen/netfront:
harden netfront against event channel storms"). On an iperf benchmark,
spurious interrupts can represent up to 50% of the interrupts.

Spurious interrupts are interrupts that are rised for nothing, there is
no work to do. This appends because the function that handles the
interrupts ("xennet_tx_buf_gc") is also called at the end of the request
path to garbage collect the responses received during the transmission
load.

The request path is doing the work that the interrupt handler should
have done otherwise. This is particurary true when there is more than
one vcpu and get worse linearly with the number of vcpu/queue.

Moreover, this problem is amplifyed by the penalty imposed by a spurious
interrupt. When an interrupt is found spurious the interrupt chip will
delay the EOI to slowdown the backend. This delay will allow more
responses to be handled by the request path and then there will be more
chance the next interrupt will not find any work to do, creating a new
spurious interrupt.

This causes performance issue. The solution here is to remove the calls
from the request path and let the interrupt handler do the processing of
the responses. This approch removes most of the spurious interrupts
(<0.05%) and also has the benefit of freeing up cycles in the request
path, allowing it to process more work, which improves performance
compared to masking the spurious interrupt one way or another.

This optimization changes a part of the code that is present since the
net frontend driver was upstreamed. There is no similar pattern in the
other xen PV drivers. Since the first commit of xen-netfront is a blob
that doesn't explain all the design choices I can only guess why this
specific mecanism was here. This could have been introduce to compensate
a slow backend at the time (maybe the backend was fixed or optimize
later) or a small queue. In 18 years, both frontend and backend gain lot
of features and optimizations that could have obsolete the feature of
reaping completions from the TX path.

Some vif throughput performance figures from a 8 vCPUs, 4GB of RAM HVM
guest(s):

Without this patch on the :
vm -> dom0: 4.5Gb/s
vm -> vm:   7.0Gb/s

Without XSA-391 patch (revert of b27d47950e):
vm -> dom0: 8.3Gb/s
vm -> vm:   8.7Gb/s

With XSA-391 and this patch:
vm -> dom0: 11.5Gb/s
vm -> vm:   12.6Gb/s

v2:
- add revewed and tested by tags
- resend with the maintainers in the recipients list

v3:
- remove Fixes tag but keep the commit ref in the explanation
- add a paragraph on why this code was here

Signed-off-by: Anthoine Bourgeois <anthoine.bourgeois@vates.tech>
Reviewed-by: Juergen Gross <jgross@suse.com>
Tested-by: Elliott Mitchell <ehem+xen@m5p.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250721093316.23560-1-anthoine.bourgeois@vates.tech>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-08-20 18:30:28 +02:00
arch x86/bugs: Avoid warning when overriding return thunk 2025-08-20 18:30:26 +02:00
block block: ensure discard_granularity is zero when discard is not supported 2025-08-15 12:14:05 +02:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: ecdsa - Harden against integer overflows in DIV_ROUND_UP() 2025-07-17 18:37:24 +02:00
Documentation Documentation: ACPI: Fix parent device references 2025-08-20 18:30:15 +02:00
drivers xen/netfront: Fix TX response spurious interrupts 2025-08-20 18:30:28 +02:00
fs tracefs: Add d_delete to remove negative dentries 2025-08-20 18:30:21 +02:00
include powerpc/thp: tracing: Hide hugepage events under CONFIG_PPC_BOOK3S_64 2025-08-20 18:30:28 +02:00
init io_uring: fix breakage in EXPERT menu 2025-08-15 12:13:32 +02:00
io_uring io_uring/poll: fix POLLERR handling 2025-07-24 08:56:23 +02:00
ipc ipc: fix to protect IPCS lookups using RCU 2025-06-27 11:11:22 +01:00
kernel PM: sleep: console: Fix the black screen issue 2025-08-20 18:30:25 +02:00
lib lib/alloc_tag: do not acquire non-existent lock in alloc_tag_top_users() 2025-07-17 18:37:14 +02:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm mm: swap: fix potential buffer overflow in setup_clusters() 2025-08-15 12:14:14 +02:00
net Bluetooth: hci_sock: Reset cookie to zero in hci_sock_free_cookie() 2025-08-20 18:30:28 +02:00
rust rust: use #[used(compiler)] to fix build and modpost with Rust >= 1.89.0 2025-07-24 08:56:37 +02:00
samples samples: mei: Fix building on musl libc 2025-08-15 12:13:38 +02:00
scripts kconfig: qconf: fix ConfigList::updateListAllforAll() 2025-08-15 12:14:03 +02:00
security securityfs: don't pin dentries twice, once is enough... 2025-08-20 18:30:21 +02:00
sound ASoC: qcom: use drvdata instead of component to keep id 2025-08-20 18:30:28 +02:00
tools selftests: netfilter: Enable CONFIG_INET_SCTP_DIAG 2025-08-20 18:30:28 +02:00
usr kbuild: hdrcheck: fix cross build with clang 2025-03-13 13:02:18 +01:00
virt KVM: Allow CPU to reschedule while setting per-page memory attributes 2025-07-17 18:37:08 +02:00
.clang-format clang-format: Update with v6.11-rc1's for_each macro list 2024-08-02 13:20:31 +02:00
.clippy.toml rust: give Clippy the minimum supported Rust version 2025-08-01 09:48:44 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore Add Jeff Kirsher to .get_maintainer.ignore 2024-03-08 11:36:54 +00:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore rust: introduce .clippy.toml 2025-03-13 13:01:42 +01:00
.mailmap mailmap: add entry for Thorsten Blum 2024-11-07 14:14:59 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Remove self from DSA entry 2024-11-03 12:52:38 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: Update Alexey Makhalov's email address 2025-05-22 14:29:46 +02:00
Makefile Linux 6.12.42 2025-08-15 12:14:14 +02:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

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 reStructuredText 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.