Re: [PATCH v2 2/5] x86/virt/tdx: Pull kexec cache flush logic into arch/x86

[H. Peter Anvin]

On April 1, 2026 8:03:02 AM PDT, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>On 3/31/26 16:04, Sean Christopherson wrote:
>> But unless the WBINVD is actually costly, why bother getting fancy?
>
>WBINVD might be the most expensive single instruction in the whole ISA.
>
>That said, I'd much rather have a potentially unnecessary WBINVD than
>miss one. The thing I'd be worried about would be something wonky like:
>
>	1. CPU offline does WBINVD
>	2. Some other TDX call gets made, dirties caches again
>	3. tdx_offline_cpu() skips WBINVD
>
>So, let's just do both for now: Do WBINVD in tdx_offline_cpu() and
>comment that it might be redundant with other things in the CPU offline
>procedure.
>
>This really needs to be solved with infrastructure and keeping data
>about the reasons for needing WBINVD, not relying on code ordering or
>fragile semantics.

It is, *by far*, the most expensive *uninterruptible* instruction in the ISA. REP string instructions can of course be arbitrarily long, but are interruptible and so don't really count.

Some MSRs used during very early (pre-OS) initialization might be even slower on some implementations, but that's not visible to Linux and no workload of any kind is running.