linux-kernel - Re: [PATCH v11 05/20] x86/virt/tdx: Add SEAMCALL infrastructure

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Date:   Thu, 8 Jun 2023 06:50:18 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Huang, Kai" <kai.huang@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Luck, Tony" <tony.luck@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "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>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v11 05/20] x86/virt/tdx: Add SEAMCALL infrastructure

On 6/7/23 17:51, Huang, Kai wrote:
> How about I add below to the changelog?
> 
> "
> The current TDX_MODULE_CALL macro handles neither #GP nor #UD.  The kernel would
> hit Oops if SEAMCALL were mistakenly made when TDX is enabled by the BIOS or
> when CPU isn't in VMX operation.  For the former, the callers could check
> platform_tdx_enabled() first, although that doesn't rule out the buggy BIOS in
> which case the kernel could still get Oops.  For the latter, the caller could
> check CR4.VMXE based on the fact that currently setting this bit and doing VMXON
> are done together when IRQ is disabled, although from hardware's perspective
> checking CR4.VMXE isn't enough.
> 
> However this could be problematic if SEAMCALL is called in the cases such as
> exception handler, NMI handler, etc, as disabling IRQ doesn't prevent any of
> them from happening.
> 
> To have a clean solution, just make the SEAMCALL always return error code by
> using EXTTABLE so the SEAMCALL can be safely called in any context.  A later
> patch will need to use SEAMCALL in the machine check handler.  There might be
> such use cases in the future too.
> "

No, that's just word salad.

SEAMCALL is like VMRESUME.  It's will be called by KVM in unsafe (VMX
off) contexts in normal operation like "reboot -f".  That means it needs
an exception handler for #UD(???).

I don't care if a bad BIOS can cause #GP.  Bad BIOS == oops.  You can
argue that even if I don't care, it's worth having a nice error message
and a common place for SEAMCALL error handling.  But it's not
functionally needed.