linux-kernel - Re: [PATCH v3 04/34] KVM: x86: hyper-v: Handle HVCALL_FLUSH_VIRTUAL_ADDRESS

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Message-ID: <YoUI03NDdMPYTiSO@google.com>
Date:   Wed, 18 May 2022 14:55:15 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v3 04/34] KVM: x86: hyper-v: Handle
 HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST{,EX} calls gently

On Wed, May 18, 2022, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> 
> > On Wed, May 18, 2022, Vitaly Kuznetsov wrote:
> >> Maxim Levitsky <mlevitsk@...hat.com> writes:
> >> > Or if using kfifo, then it can contain plain u64 items, which is even more natural.
> >> >
> >> 
> >> In the next version I switch to fifo and get rid of 'flush_all' entries
> >> but instead of a boolean I use a 'magic' value of '-1' in GVA. This way
> >> we don't need to synchronize with the reader and add any special
> >> handling for the flag.
> >
> > Isn't -1 theoretically possible?  Or is wrapping not allowed?  E.g. requesting a
> > flush for address=0xfffffffffffff000, count = 0xfff will yield -1 and doesn't
> > create any illegal addresses in the process.
> >
> 
> Such an error would just lead to KVM flushing the whole guest address
> space instead of flushing 4096 pages starting with 0xfffffffffffff000
> but over-flushing is always architecturally correct, isn't it?

Oh, duh.  Yeah, flushing everything is totally ok.  Maybe just add a comment above
the #define for the magic value calling out that corner case and why it's ok?