linux-kernel - Re: [PATCH v12 05/22] x86/virt/tdx: Add SEAMCALL infrastructure

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Date:   Tue, 27 Jun 2023 17:19:06 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        isaku.yamahata@...il.com
Subject: Re: [PATCH v12 05/22] x86/virt/tdx: Add SEAMCALL infrastructure

On Tue, Jun 27, 2023 at 10:28:20AM +0000,
"Huang, Kai" <kai.huang@...el.com> wrote:

> On Tue, 2023-06-27 at 12:48 +0300, kirill.shutemov@...ux.intel.com wrote:
> > On Tue, Jun 27, 2023 at 02:12:35AM +1200, Kai Huang wrote:
> > > +/*
> > > + * Wrapper of __seamcall() to convert SEAMCALL leaf function error code
> > > + * to kernel error code.  @seamcall_ret and @out contain the SEAMCALL
> > > + * leaf function return code and the additional output respectively if
> > > + * not NULL.
> > > + */
> > > +static int __always_unused seamcall(u64 fn, u64 rcx, u64 rdx, u64 r8, u64 r9,
> > > +				    u64 *seamcall_ret,
> > > +				    struct tdx_module_output *out)
> > > +{
> > > +	u64 sret;
> > > +	int cpu;
> > > +
> > > +	/* Need a stable CPU id for printing error message */
> > > +	cpu = get_cpu();
> > > +	sret = __seamcall(fn, rcx, rdx, r8, r9, out);
> > > +	put_cpu();
> > > +
> > > +	/* Save SEAMCALL return code if the caller wants it */
> > > +	if (seamcall_ret)
> > > +		*seamcall_ret = sret;
> > > +
> > > +	switch (sret) {
> > > +	case 0:
> > > +		/* SEAMCALL was successful */
> > > +		return 0;
> > > +	case TDX_SEAMCALL_VMFAILINVALID:
> > > +		pr_err_once("module is not loaded.\n");
> > > +		return -ENODEV;
> > > +	default:
> > > +		pr_err_once("SEAMCALL failed: CPU %d: leaf %llu, error 0x%llx.\n",
> > > +				cpu, fn, sret);
> > > +		if (out)
> > > +			pr_err_once("additional output: rcx 0x%llx, rdx 0x%llx, r8 0x%llx, r9 0x%llx, r10 0x%llx, r11 0x%llx.\n",
> > > +					out->rcx, out->rdx, out->r8,
> > > +					out->r9, out->r10, out->r11);
> > 
> > This look excessively noisy.
> > 
> > Don't we have SEAMCALL leafs that can fail in normal situation? Like
> > TDX_OPERAND_BUSY error code that indicate that operation likely will
> > succeed on retry.
> 
> For TDX module initialization TDX_OPERAND_BUSY cannot happen.  KVM may have
> legal cases that BUSY can happen, e.g., KVM's TDP MMU supports handling faults
> concurrently on different cpus, but that is still under discussion.  Also KVM
> tends to use __seamcall() directly:
> 
> https://lore.kernel.org/lkml/3c2c142e14a04a833b47f77faecaa91899b472cd.1678643052.git.isaku.yamahata@intel.com/
> 
> I guess KVM doesn't want to print message in all cases as you said, but for
> module initialization is fine.  Those error messages are useful in case
> something goes wrong, and printing them in seamcall() helps to reduce the code
> to print in all callers.

That's right.  KVM wants to do its own error handling and error messaging.  Its
requirement is different from TDX module initialization. I didn't see much
benefit to unify the function.
-- 
Isaku Yamahata <isaku.yamahata@...il.com>