linux-kernel - Re: [PATCH] x86/tdx: Handle load_unaligned

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Message-ID: <YoQnQIfX8GuOgKqH@google.com>
Date:   Tue, 17 May 2022 22:52:48 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, ak@...ux.intel.com,
        dan.j.williams@...el.com, david@...hat.com, hpa@...or.com,
        thomas.lendacky@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/tdx: Handle load_unaligned_zeropad() page-cross to a
 shared page

On Tue, May 17, 2022, Sean Christopherson wrote:
> On Tue, May 17, 2022, Dave Hansen wrote:
> > On 5/17/22 13:17, Kirill A. Shutemov wrote:
> > >>> Given that we had to adjust IP in handle_mmio() anyway, do you still think
> > >>> "ve->instr_len = 0;" is wrong? I dislike ip_adjusted more.
> > >> Something is wrong about it.
> > >>
> > >> You could call it 've->instr_bytes_to_handle' or something. Then it
> > >> makes actual logical sense when you handle it to zero it out.  I just
> > >> want it to be more explicit when the upper levels need to do something.
> > >>
> > >> Does ve->instr_len==0 both when the TDX module isn't providing
> > >> instruction sizes *and* when no handling is necessary?  That seems like
> > >> an unfortunate logical multiplexing of 0.
> > > For EPT violation, ve->instr_len has *something* (not zero) that doesn't
> > > match the actual instruction size. I dig out that it is filled with data
> > > from VMREAD(0x440C), but I don't know where is the ultimate origin of the
> > > data.
> > 
> > The SDM has a breakdown:
> > 
> > 	27.2.5 Information for VM Exits Due to Instruction Execution
> > 
> > I didn't realize it came from VMREAD.  I guess I assumed it came from
> > some TDX module magic.  Silly me.
> > 
> > The SDM makes it sound like we should be more judicious about using
> > 've->instr_len' though.  "All VM exits other than those listed in the
> > above items leave this field undefined."  Looking over
> > virt_exception_kernel(), we've got five cases from CPU instructions that
> > cause unconditional VMEXITs:

Ideally, what the SDM says wouldn't matter at all.  The TDX module spec really
should be the authorative source in this case, but it just punts to the SDM:

  The 32-bit value that would have been saved into the VMCS as VM-exit instruction
  length if a legacy VM exit had occurred instead of the virtualization exception.

Even if the TDX spec wants to punt to the SDM, it would save a lot of headache and
SDM reading if it also said something to the effect of:

  The INSTRUCTION_LENGTH and INSTRUCTION_INFORMATION fields are valid for all
  #VEs injected by the Intel TDX Module.  The fields are undefined for #VEs
  injected by the CPU due to EPT Violations.