linux-kernel - Re: [PATCH v5 07/12] x86/traps: Add #VE support for TDX guest

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Date:   Thu, 2 Sep 2021 08:24:53 -0700
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter H Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 07/12] x86/traps: Add #VE support for TDX guest

On 8/24/21 10:46 AM, Borislav Petkov wrote:
> On Tue, Aug 24, 2021 at 10:32:13AM -0700, Kuppuswamy, Sathyanarayanan wrote:
>> Mainly chose it avoid future name conflicts with KVM (tdx) calls. But
> 
> What name conflicts with KVM calls? Please explain.

Currently there are no name conflicts. But in our initial submissions (RFC v?)
we had some conflicts in functions like (tdx_get_tdreport() and
tdx_get_quote()).

Since it is no longer true and "tdg" is not a favorite prefix, I will
rename tdg -> tdx in next submission.

> 
>> It is required to handle #VE exceptions raised by unhandled MSR
>> read/writes.
> 
> Example? Please elaborate.

If MSR read/write failed in tdx_handle_virtualization_exception(), it will
return non zero return value which in turn will trigger ve_raise_fault().

If we don't call fixup_exception() for such case, it will trigger oops
and eventually panic in TDX. For MSR read/write failures we don't want
to panic.

#VE MSR read/write
  -> exc_virtualization_exception()
     -> tdx_handle_virtualization_exception()
        ->tdx_write_msr_safe()
     -> ve_raise_fault
        -> fixup_exception()

> 
>> Ok. I can check it. But there is only one statement after this call.
>> So it may not be very helpful.
> 
> Looking at die_addr(), that calls the die notifier too. So do you
> even *have* to call it here with VEFSTR? As yo say, there's only one
> statement after that call and box is dead in the water after that so why
> even bother...

Reason for calling die_addr() is to trigger oops for failed #VE handling, which
is desirable for TDX. Also sending die notification may be useful for debuggers.

This sequence of calls are similar to exc_general_protection().

> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer