linux-kernel - Re: [RFC v1 09/26] x86/tdx: Handle CPUID via #VE

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Date:   Sun, 7 Feb 2021 23:29:13 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v1 09/26] x86/tdx: Handle CPUID via #VE

On Sun, Feb 07, 2021 at 08:01:50AM -0800, Dave Hansen wrote:
> On 2/7/21 6:13 AM, Kirill A. Shutemov wrote:
> >>> +       /* Allow to pass R10, R11, R12, R13, R14 and R15 down to the VMM */
> >>> +       rcx = BIT(10) | BIT(11) | BIT(12) | BIT(13) | BIT(14) | BIT(15);
> >>> +
> >>> +       asm volatile(TDCALL
> >>> +                       : "=a"(ret), "=r"(r10), "=r"(r11), "=r"(r12), "=r"(r13),
> >>> +                         "=r"(r14), "=r"(r15)
> >>> +                       : "a"(TDVMCALL), "r"(rcx), "r"(r10), "r"(r11), "r"(r12),
> >>> +                         "r"(r13)
> >>> +                       : );
> >> Some "+" constraints would make this simpler.  But I think you should
> >> factor the TDCALL helper out into its own function.
> > Factor out TDCALL into a helper is tricky: different TDCALLs have
> > different list of registers passed to VMM.
> 
> Couldn't you just have one big helper that takes *all* the registers
> that get used in any TDVMCALL and sets all the rcx bits?  The users
> could just pass 0's for the things they don't use.
> 
> Then you've got the ugly inline asm in one place.  It also makes it
> harder to screw up the 'rcx' mask and end up passing registers you
> didn't want into a malicious VMM.

For now we only pass down R10-R15, but the interface allows to pass down
much wider set of registers, including XMM. How far do we want to get it?

-- 
 Kirill A. Shutemov