Re: [PATCH v1] x86/cpu/cacheinfo: fix panic on Ryzen Mobile 7x40 series in VBox VM

[Christian Lamparter]

Heads up!

Oracle addressed this with their virtualbox 7.0.12 release.
If you are affected, try it!

On Saturday, September 2, 2023 12:19:11 AM CEST Christian Lamparter wrote:
> Ryzen Mobile 7x40 chips experience an early kernel fault
> (division by zero) due to nonexistent input validation
> from CPUID data in amd_cpuid4().
> 
> This error was first reported on reddit [0] for a
> "AMD Ryzen 9 7940HS w/ Radeon 780M Graphics", but since then
> the bug has been reproduced on a "AMD Ryzen 7 7840HS" Laptop
> too.
> 
> [...]
> 
> The reason why this is happening is because the Ryzen CPU reports in its
> CPUID 80000006 edx register (which contains the L3 Cache Information)
> the value "00009000". This magic value means according to AMD's
> "AMD64 Architecture Programmer's Manual Volume 3" Table E-4.
> "L2/L3 Cache and TLB Associativity Field Encoding":
> 
> | Value for all fields should be determinded from Fn8000_001D.
> 
> (This means to look in cpuid(0x8000001D,...) instead.)
> 
> So, amd_cpuid4() is missing this special case. A case which has been
> present - according to this AMD Community post [1] - since
> Zen 2/Ryzen 3000 Series.
> 
> But wait, why is this only happening with Linux' when running under
> VirtualBox with these Ryzen Mobile CPUs and not when running natively?
> [...]