Re: [PATCH 13/25] KVM: TDX: create/destroy VM structure

[Tony Lindgren <tony.lindgren@linux.intel.com>]

On Wed, Aug 21, 2024 at 12:23:42AM +0000, Edgecombe, Rick P wrote:
> On Mon, 2024-08-19 at 18:09 +0300, Nikolay Borisov wrote:
> > > +/*
> > > + * Some SEAMCALLs acquire the TDX module globally, and can fail with
> > > + * TDX_OPERAND_BUSY.  Use a global mutex to serialize these SEAMCALLs.
> > > + */
> > > +static DEFINE_MUTEX(tdx_lock);
> > 
> > The way this lock is used is very ugly. So it essentially mimics a lock 
> > which already lives in the tdx module. So why not simply gracefully 
> > handle the TDX_OPERAND_BUSY return value or change the interface of the 
> > module (yeah, it's probably late for this now) so expose the lock. This 
> > lock breaks one of the main rules of locking - "Lock data and not code"
> 
> Hmm, we would have to make SEAMCALLs to spin on that lock, where as mutexes can
> sleep. I suspect that is where it came from. But we are trying to make the code
> simple and obviously correct and add optimizations later. This might fit that
> pattern, especially since it is just used during VM creation and teardown.

For handling the busy retries for SEAMCALL callers, we could just use
iopoll.h read_poll_timeout(). I think it can handle toggling the resume
bit while looping, need to test that though. See for example the
smp_func_do_phymem_cache_wb() for toggling the resume variable.

The overhead of a SEAMCALL may not be that bad in the retry case.

Regards,

Tony