[Bug 1835809] Re: AMD Ryzen 3000 series fails to boot

[Balint Reczey]

** Description changed:

+ [Impact]
+ 
+  * Systems with AMD Ryzen 3000 series CPUs don't boot.
+ 
+ [Test Case]
+ 
+  * Boot with fixed systemd on an AMD Ryzen 3000 series system.
+ 
+ [Regression Potential]
+ 
+  * The fix itself is very small, it ignores known to be faulty random
+ values returned by the rdrand instruction and use a different random
+ source. Those values can still be returned by a properly working rdrand
+ implementation in 2 in 2^32 cases on 32 bit arches and in 2 in 2^64
+ cases on 64 bit arches, but the fallback to the other random source
+ ensures that in those rare occasions a random number can be generated.
+ 
+ [Original Bug Text]
+ 
  On the new AMD Ryzen 3000 series CPUs, there is an issue with systemd
  preventing the boot process from completing. This issue does not affect
  the older systemd version in 18.04, but affects the 19.04 version.
  
  Here is a screenshot showing what happens:
  https://www.phoronix.net/image.php?id=ryzen-3700x-3900x-linux&image=amd_zen2_14_show
  
  I am currently testing a patch to systemd, derived from this pull request:
  https://github.com/systemd/systemd/pull/12536
  
  This is a high severity issue, as I do not believe there is no potential
  workaround without either a firmware update or an ISO respin.
  
  I have attached a rebase of the potential patch on the current 19.04
  version of systemd for reference. I will provide more details after
  testing.

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https://bugs.launchpad.net/bugs/1835809

Title:
  AMD Ryzen 3000 series fails to boot

Status in systemd package in Ubuntu:
  In Progress
Status in systemd source package in Disco:
  In Progress

Bug description:
  [Impact]

   * Systems with AMD Ryzen 3000 series CPUs don't boot.

  [Test Case]

   * Boot with fixed systemd on an AMD Ryzen 3000 series system.

  [Regression Potential]

   * The fix itself is very small, it ignores known to be faulty random
  values returned by the rdrand instruction and use a different random
  source. Those values can still be returned by a properly working
  rdrand implementation in 2 in 2^32 cases on 32 bit arches and in 2 in
  2^64 cases on 64 bit arches, but the fallback to the other random
  source ensures that in those rare occasions a random number can be
  generated.

  [Original Bug Text]

  On the new AMD Ryzen 3000 series CPUs, there is an issue with systemd
  preventing the boot process from completing. This issue does not
  affect the older systemd version in 18.04, but affects the 19.04
  version.

  Here is a screenshot showing what happens:
  https://www.phoronix.net/image.php?id=ryzen-3700x-3900x-linux&image=amd_zen2_14_show

  I am currently testing a patch to systemd, derived from this pull request:
  https://github.com/systemd/systemd/pull/12536

  This is a high severity issue, as I do not believe there is no
  potential workaround without either a firmware update or an ISO
  respin.

  I have attached a rebase of the potential patch on the current 19.04
  version of systemd for reference. I will provide more details after
  testing.

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