Re: [PATCH v11 19/20] x86/mce: Improve error log of kernel space TDX #MC due to erratum

["Huang, Kai" <kai.huang@intel.com>]

On Fri, 2023-06-09 at 16:17 +0300, kirill.shutemov@linux.intel.com wrote:
> On Mon, Jun 05, 2023 at 02:27:32AM +1200, Kai Huang wrote:
> > The first few generations of TDX hardware have an erratum.  Triggering
> > it in Linux requires some kind of kernel bug involving relatively exotic
> > memory writes to TDX private memory and will manifest via
> > spurious-looking machine checks when reading the affected memory.
> > 
> > == Background ==
> > 
> > Virtually all kernel memory accesses operations happen in full
> > cachelines.  In practice, writing a "byte" of memory usually reads a 64
> > byte cacheline of memory, modifies it, then writes the whole line back.
> > Those operations do not trigger this problem.
> > 
> > This problem is triggered by "partial" writes where a write transaction
> > of less than cacheline lands at the memory controller.  The CPU does
> > these via non-temporal write instructions (like MOVNTI), or through
> > UC/WC memory mappings.  The issue can also be triggered away from the
> > CPU by devices doing partial writes via DMA.
> > 
> > == Problem ==
> > 
> > A partial write to a TDX private memory cacheline will silently "poison"
> > the line.  Subsequent reads will consume the poison and generate a
> > machine check.  According to the TDX hardware spec, neither of these
> > things should have happened.
> > 
> > To add insult to injury, the Linux machine code will present these as a
> > literal "Hardware error" when they were, in fact, a software-triggered
> > issue.
> > 
> > == Solution ==
> > 
> > In the end, this issue is hard to trigger.  Rather than do something
> > rash (and incomplete) like unmap TDX private memory from the direct map,
> > improve the machine check handler.
> > 
> > Currently, the #MC handler doesn't distinguish whether the memory is
> > TDX private memory or not but just dump, for instance, below message:
> > 
> >  [...] mce: [Hardware Error]: CPU 147: Machine Check Exception: f Bank 1: bd80000000100134
> >  [...] mce: [Hardware Error]: RIP 10:<ffffffffadb69870> {__tlb_remove_page_size+0x10/0xa0}
> >  	...
> >  [...] mce: [Hardware Error]: Run the above through 'mcelog --ascii'
> >  [...] mce: [Hardware Error]: Machine check: Data load in unrecoverable area of kernel
> >  [...] Kernel panic - not syncing: Fatal local machine check
> > 
> > Which says "Hardware Error" and "Data load in unrecoverable area of
> > kernel".
> > 
> > Ideally, it's better for the log to say "software bug around TDX private
> > memory" instead of "Hardware Error".  But in reality the real hardware
> > memory error can happen, and sadly such software-triggered #MC cannot be
> > distinguished from the real hardware error.  Also, the error message is
> > used by userspace tool 'mcelog' to parse, so changing the output may
> > break userspace.
> > 
> > So keep the "Hardware Error".  The "Data load in unrecoverable area of
> > kernel" is also helpful, so keep it too.
> > 
> > Instead of modifying above error log, improve the error log by printing
> > additional TDX related message to make the log like:
> > 
> >   ...
> >  [...] mce: [Hardware Error]: Machine check: Data load in unrecoverable area of kernel
> >  [...] mce: [Hardware Error]: Machine Check: Memory error from TDX private memory. May be result of CPU erratum.
> 
> The message mentions one part of issue -- CPU erratum -- but misses the
> other required part -- kernel bug that makes kernel access the memory it
> not suppose to.
> 

How about below?

"Memory error from TDX private memory. May be result of CPU erratum caused by
kernel bug."