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

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Message-ID: <ZtGTj40bKDVss_Mv@tlindgre-MOBL1>
Date: Fri, 30 Aug 2024 12:40:31 +0300
From: Tony Lindgren <tony.lindgren@...ux.intel.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "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>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"nik.borisov@...e.com" <nik.borisov@...e.com>,
	"sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
	"Wang, Wei W" <wei.w.wang@...el.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>
Subject: Re: [PATCH 13/25] KVM: TDX: create/destroy VM structure

On Wed, Aug 21, 2024 at 04:52:14PM +0000, Edgecombe, Rick P wrote:
> On Wed, 2024-08-21 at 08:39 +0300, Tony Lindgren wrote:
> > > 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.
> 
> Nice. It seems worth trying to me.

To recap on this, using iopoll for smp_func_do_phymem_cache_wb() would look like:

static void smp_func_do_phymem_cache_wb(void *unused)
{
	u64 status = 0;
	int err;

	err = read_poll_timeout_atomic(tdh_phymem_cache_wb, status,
				       status != TDX_INTERRUPTED_RESUMABLE,
				       1, 1000, 0, !!status);
	if (WARN_ON_ONCE(err)) {
		pr_err("TDH_PHYMEM_CACHE_WB timed out: 0x%llx\n", status);
		return;
	}
	...
}

For the retry flag toggling with the !!status, I think it's best to add a TDX
specific tdx_read_poll_timeout_atomic() macro.

Regards,

Tony