Messages in this thread | | | Subject | Re: [PATCH v6 11/11] x86/tdx: Handle CPUID via #VE | From | "Kuppuswamy, Sathyanarayanan" <> | Date | Fri, 3 Sep 2021 12:14:48 -0700 |
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On 9/3/21 11:35 AM, Dave Hansen wrote: > On 9/3/21 10:28 AM, Kuppuswamy Sathyanarayanan wrote: >> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> >> >> TDX has three classes of CPUID leaves: some CPUID leaves are always >> handled by the CPU, others are handled by the TDX module, and some >> others are handled by the VMM. Since the VMM cannot directly intercept >> the instruction these are reflected with a #VE exception to the guest, >> which then converts it into a hypercall to the VMM, or handled >> directly. > > Does this patch do any of the "handled directly" leaves? If not, why > mention it?
It was added to give more information about CPUID leaves handling. Since it has nothing to do with this patch, I can remove it.
> > It would also be nice to mention that this applies to both kernel and > userspace use of CPUID. It talks a bit about early kernel use, which > makes it seem like this is kernel-only. > > I also think it's a mistake to talk about TDX-module handling. For > *this* patch, it doesn't matter. > > Here's a reformatted replacement changelog: > > -- > > When running virtualized, the CPUID instruction is handled differently > based on the leaf being accessed. The behavior depends only on on the > leaf and applies equally to both kernel/ring-0 and userspace/ring-3 > execution of CPUID. Historically, there are two basic classes: > > * Leaves handled transparently to the guest > * Leaves handled by the VMM > > In a typical guest without TDX, "handled by the VMM" leaves cause a > VMEXIT. TDX replaces these VMEXITs with a #VE exception in the guest. > The guest typically handles the #VE by making a hypercall to the VMM. > > The TDX module spec talks about a few more classes of CPUID handling. > But, for the purposes of this patch, the "handled transparently" CPUID > leaves are all lumped together because the guest handling is the same. > > --
Thanks. I will use above commit log in next version.
> >> The TDX module specification [1], sec 16.2 has a full list of CPUID > > ^ I think we can spare the extra four bytes to make "sec" -> > "section". > > I also opened up the pdf from [1] an searched for "16.2". I found: > > 16.2. Branch Prediction Side Channel Attacks Mitigation > Mechanisms > > There is, however, a: > > 18.2. CPUID Virtualization > > section. Did you, perhaps, mean to reference that instead?
It looks like I have been using previous version of the TDX module spec (Sep 2020). In the newer version, it is changed to 18.2.
To avoid confusion I will use the section title for reference. > > Which kinda argues for not using these section numbers at *all*. > Perhaps you should just mention the section titles, as they're obviously > less volatile. That's probably a comment that applies to *ALL* of your > changelogs across *ALL* TDX patches. This just proves that the section > numbers are worthless.
Makes sense. I will fix it in all TDX patch series.
> >> leaves which are handled natively or by the TDX module. Only unknown > > Just in terms of nice writing, it would be great to use the same > language when you refer to the same concept. Earlier you called this > "handled by the CPU". But, here you refer to it as being "handled > natively". Neither is wrong, but this puts a burden on the reader to > make a connection rather than doing it for them as the writer.
Ok. I will keep this in mind for future submissions. For this patch your commit log refactor fixes this issue.
> >> CPUIDs are handled by the #VE method. In practice this typically only >> applies to the hypervisor-specific CPUIDs unknown to the native CPU. >> >> Therefore there is no risk of causing this in early CPUID code which >> runs before the #VE handler is set up because it will never access >> those exotic CPUID leaves. > > This never actually makes a transition from background to telling what > the patch does. I think this needs at least this: > > Allow the the #VE code to handle any CPUID leaves which cause a > #VE. Unconditionally make a TDCALL to the VMM. > >> diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c >> index 5c52dde4a5fd..c65c117aff5f 100644 >> --- a/arch/x86/kernel/tdx.c >> +++ b/arch/x86/kernel/tdx.c >> @@ -150,6 +150,21 @@ static int tdx_write_msr_safe(unsigned int msr, unsigned int low, >> return ret ? -EIO : 0; >> } >> >> +static u64 tdx_handle_cpuid(struct pt_regs *regs) >> +{ >> + struct tdx_hypercall_output out = {0}; >> + u64 ret; >> + >> + ret = _tdx_hypercall(EXIT_REASON_CPUID, regs->ax, regs->cx, 0, 0, &out); >> + >> + regs->ax = out.r12; >> + regs->bx = out.r13; >> + regs->cx = out.r14; >> + regs->dx = out.r15; > > This probably needs a comment about why this is shuffling registers > around like this.
I will add the ABI details here and also spell out what we are getting in R12-R15 registers.
>
-- Sathyanarayanan Kuppuswamy Linux Kernel Developer
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