linux-kernel - Re: [RFC v2-fix-v4 1/1] x86/tdx: Skip WBINVD instruction for TDX guest

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Message-ID: <390d6712-35b5-e52a-fb32-205844bad4ef@linux.intel.com>
Date:   Tue, 8 Jun 2021 21:40:42 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2-fix-v4 1/1] x86/tdx: Skip WBINVD instruction for TDX
 guest


>> KVM only turns it into a noop if there is no VT-d, because with VT-d you
>> might need it to turn mappings into uncached and vice versa.
> Wow, I found the kvm_arch_register_noncoherent_dma() stuff.  That's horrifying.  What's it for?  e

e.g. if you want to run a GPU it really needs some uncached memory. Same 
is true for other more complex devices.

Now modern Linux of course will be preferring CLFLUSH instead for the 
conversion, but there are old versions that preferred WBINVD.

I don't think it's a DoS, as long as you're not too picky about 
latencies on the host.

-Andi