linux-kernel - Re: [PATCH 08/10] x86/tdx: Unify TDX_HYPERCALL and TDX_MODULE

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Message-ID: <0cb00d72e7ebb0969a7ac10f6ade9eea20deaef5.camel@intel.com>
Date:   Tue, 18 Jul 2023 10:32:20 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "Hansen, Dave" <dave.hansen@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [PATCH 08/10] x86/tdx: Unify TDX_HYPERCALL and TDX_MODULE_CALL
 assembly

On Mon, 2023-07-17 at 07:58 +0000, Huang, Kai wrote:
> > On 17.07.23 г. 9:35 ч., Huang, Kai wrote:
> > > 
> > > > > +/* Called from __tdx_hypercall() for unrecoverable failure */
> > > > > +static noinstr void __tdx_hypercall_failed(void) {
> > > > > +	instrumentation_begin();
> > > > > +	panic("TDVMCALL failed. TDX module bug?"); }
> > > > 
> > > > So what's the deal with this instrumentation here. The instruction is
> > > > noinstr, so you want to make just the panic call itself
> > > > instrumentable?, if so where's the instrumentation_end() cal;?No
> > > > instrumentation_end() call. Actually is this complexity really worth it for the
> > failure case?
> > > > 
> > > > AFAICS there is a single call site for __tdx_hypercall_failed so why
> > > > noot call panic() directly ?
> > > 
> > > W/o this patch, the __tdx_hypercall_failed() is called from the
> > > TDX_HYPERCALL assembly, which is in .noinstr.text, and
> > > 'instrumentation_begin()' was needed to avoid the build warning I suppose.
> > > 
> > > However now with this patch __tdx_hypercall_failed() is called from
> > > __tdx_hypercall() which is a C function w/o 'noinstr' annotation, thus
> > > I believe
> > > instrumentation_begin() and 'noinstr' annotation are not needed anymore.
> > > 
> > > I didn't notice this while moving this function around and my kernel
> > > build test didn't warn me about this.  I'll change in next version.
> > > 
> > > In fact, perhaps this patch perhaps is too big for review.  I will
> > > also try to split it to smaller ones.
> > 
> > Can't you simply call panic() directly? Less going around the code while someone
> > is reading it?
> 
> I can and will do.

After rebasing to the latest TDX code, I found we should keep the
__tdx_hypercall_failed().  The reason is both the core-kernel (vmlinux) and the
compressed code need the __tdx_hypercall() implementation.  Implementing the
__tdx_hypercall_failed() in both core-kernel and compressed code separately
allows the __tdx_hypercall() to be shared by both code, otherwise both of them
need to implement their own __tdx_hypercall().

Note __tdx_hypercall_failed() in the vmlinux calls panic(), but the one in the
compressed code calls error().