linux-kernel - RE: [patch 5/6] x86/fpu: Provide fpu_update_guest

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Date:   Tue, 14 Dec 2021 16:40:29 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Wang, Wei W" <wei.w.wang@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Jing Liu <jing2.liu@...ux.intel.com>,
        "Zhong, Yang" <yang.zhong@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Sean Christoperson <seanjc@...gle.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Subject: RE: [patch 5/6] x86/fpu: Provide fpu_update_guest_xcr0/xfd()

On Tue, Dec 14 2021 at 15:09, Wei W. Wang wrote:
> On Tuesday, December 14, 2021 10:50 AM, Thomas Gleixner wrote:
>> + * Return: 0 on success, error code otherwise  */ int
>> +__fpu_update_guest_features(struct fpu_guest *guest_fpu, u64 xcr0, u64
>> +xfd) {
>
> I think there would be one issue for the "host write on restore" case.
> The current QEMU based host restore uses the following sequence:
> 1) restore xsave
> 2) restore xcr0
> 3) restore XFD MSR

This needs to be fixed. Ordering clearly needs to be:

  XFD, XCR0, XSTATE

> At the time of "1) restore xsave", KVM already needs fpstate expansion
> before restoring the xsave data.

> So the 2 APIs here might not be usable for this usage.
> Our current solution to fpstate expansion at KVM_SET_XSAVE (i.e. step 1) above) is:
>
> kvm_load_guest_fpu(vcpu);
> guest_fpu->realloc_request = realloc_request;
> kvm_put_guest_fpu(vcpu);
>
> "realloc_request" above is generated from the "xstate_header" received from userspace.

That's a horrible hack. Please fix the ordering in QEMU. Trying to
accomodate for nonsensical use cases in the kernel is just wrong.

That's like you expect the following to work:

       u8 *p = mmap(NULL, 4096, ....);

       p[8192] = x;

It rightfully explodes in your face and you can keep the pieces.

Having ordering constraints vs. these 3 involved parts is just sensible.

Thanks,

        tglx