Messages in this thread | | | Subject | Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX guest | From | Andi Kleen <> | Date | Tue, 25 May 2021 18:09:21 -0700 |
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On 5/24/2021 8:40 PM, Dan Williams wrote: > On Mon, May 24, 2021 at 8:27 PM Andi Kleen <ak@linux.intel.com> wrote: >> >> On 5/24/2021 7:49 PM, Dan Williams wrote: >>> On Mon, May 24, 2021 at 7:13 PM Andi Kleen <ak@linux.intel.com> wrote: >>> [..] >>>>> ...to explicitly error out a wbinvd use case before data is altered >>>>> and wbinvd is needed. >>>> I don't see any point of all of this. We really just want to be the same >>>> as KVM. Not get into the business of patching a bazillion sub systems >>>> that cannot be used in TDX anyways. >>> Please let's not start this patch off with dubious claims of safety >>> afforded by IgnorePAT. Instead make the true argument that wbinvd is >>> known to be problematic in guests >> That's just another reason to not support WBINVD, but I don't think it's >> the main reason. The main reason is that it is simply not needed, unless >> you do DMA in some form. >> >> (and yes I consider direct mapping of persistent memory with a complex >> setup procedure a form of DMA -- my guess is that the reason that it >> works in KVM is that it somehow activates the DMA code paths in KVM) > No, it doesn't. Simply no one has tried to pass through the security > interface of bare metal nvdimm to a guest, or enabled the security > commands in a virtualized nvdimm.
Maybe a better term would be "external side effects". If you have something in IO domain which can notice a difference.
> If a guest supports a memory map it supports PMEM I struggle to see DMA anywhere in that equation.
Okay if that's happen to a TDX guest we have to start emulate WBINVD. But right now we don't need it.
I guess we can add a comment that says
"if someone wants to implement NVDIMM secure delete they would also need to implement this new hypercall"
> >> IMNSHO that's the true reason. > I do see why it would be attractive if IgnorePAT was a solid signal to > ditch wbinvd support. However, it simply isn't, and to date nothing > has cared trip over that gap.
I think we're getting into angels on a pinhead here.
The key point is that current TDX does not need WBINVD. I believe we agree on that.
-Andi
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