linux-kernel - Re: [PATCH v3 07/12] x86/tdx: Make TDX_HYPERCALL asm similar to TDX_MODULE

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Date:   Thu, 3 Aug 2023 14:45:46 +0300
From:   "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "Hansen, Dave" <dave.hansen@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "n.borisov.lkml@...il.com" <n.borisov.lkml@...il.com>
Subject: Re: [PATCH v3 07/12] x86/tdx: Make TDX_HYPERCALL asm similar to
 TDX_MODULE_CALL

On Thu, Jul 27, 2023 at 11:05:35PM +0000, Huang, Kai wrote:
> On Thu, 2023-07-27 at 20:10 +0300, kirill.shutemov@...ux.intel.com wrote:
> > On Wed, Jul 26, 2023 at 11:25:09PM +1200, Kai Huang wrote:
> > > 
> > > Remove the __tdx_hypercall_ret() as __tdx_hypercall() already does so.
> > 
> > Hm. So we now update struct on all VMCALLs. Is it a good idea? 
> > 
> 
> Do you mean we "unconditionally save output registers to  the structure", right?
> 
> > We give
> > more control to VMM where it is not needed. 
> > 
> 
> I don't quite follow this.  Can you elaborate?
> 
> Do you worry about VMM being malicious and putting malicious values to the
> registers?

Yes. Caller of the hypercall may expect that the register is in-only and
re-use the field for other stuff. And it would work until VMM decide
otherwise.

> > I would rather keep the struct
> > read-only where possible.
> > 
> 
> We can achieve this if there's a clean way to do, but I don't see that.

Keep _ret() and non-_ret() versions?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov