Intel's Graphics Driver DoS Fix Last Week Has Hurt Power Consumption

While the patches overnight about "substantial" improvement in power usage for Intel graphics on Linux were exciting on first look, it's less so now as it turns out last week's graphics driver security fixes is what regressed the Intel graphics power-savings.

During last Tuesday's round of Intel security disclosures where there was a fix for denial of service in the Intel graphics driver, it turns out that the CVE-2019-0154 fix is what regressed power usage. The potential Denial of Service vulnerability was about unprivileged users being able to cause a DoS by reading select memory regions when the graphics hardware is in certain low-power states.

That security fix keeps the Intel hardware out of its deepest C-states and that in turn can waste "1-2 Watts" while the new power-savings patches recovers from that inefficiency.

On an Intel Core i7 8550U laptop, sure enough with the latest Linux 5.4 kernel the power consumption was spiking about ~1 Watt higher compared to the same system with a kernel from last week prior to those fixes.

That graph above was while idling when the hardware should drop to its lower power states. When looking at the performance under various browser tests, the latest kernel was consuming about a half Watt more on this Dell XPS laptop:

And that slight increase in power usage is on top of degraded performance due to last week's woes:



More details via this result file.

With Linux 5.4.0 due to be released this coming Sunday, it remains to be seen if these Intel graphics power management fixes will work it into that kernel release as a last-minute fix or be held off for a point release / additional testing.