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

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Message-ID: <ZsV9qouTem-ynGJA@tlindgre-MOBL1>
Date: Wed, 21 Aug 2024 08:39:54 +0300
From: Tony Lindgren <tony.lindgren@...ux.intel.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"nik.borisov@...e.com" <nik.borisov@...e.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>,
	"Huang, Kai" <kai.huang@...el.com>,
	"Li, Xiaoyao" <xiaoyao.li@...el.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"Wang, Wei W" <wei.w.wang@...el.com>
Subject: Re: [PATCH 13/25] KVM: TDX: create/destroy VM structure

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