linux-kernel - Re: [CFT PATCH v2 0/2] KVM: support XSAVES usage in the host

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Message-Id: <400361E2-995A-44B4-B35A-0D35793B49DD@gmail.com>
Date:	Tue, 25 Nov 2014 16:05:34 +0200
From:	Nadav Amit <nadav.amit@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kvm list <kvm@...r.kernel.org>,
	Nadav Amit <namit@...technion.ac.il>, hpa@...ux.intel.com
Subject: Re: [CFT PATCH v2 0/2] KVM: support XSAVES usage in the host


> On Nov 25, 2014, at 12:36, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> 
> 
> On 25/11/2014 11:13, Wanpeng Li wrote:
>> Hi Paolo,
>> On Mon, Nov 24, 2014 at 05:43:32PM +0100, Paolo Bonzini wrote:
>>> The first patch ensures that XSAVES is not exposed in the guest until
>>> we emulate MSR_IA32_XSS.  The second exports XSAVE data in the correct
>>> format.
>>> 
>>> I tested these on a non-XSAVES system so they should not be completely
>>> broken, but I need some help.  I am not even sure which XSAVE states
>>> are _not_ enabled, and thus compacted, in Linux.
>>> 
>>> Note that these patches do not add support for XSAVES in the guest yet,
>>> since MSR_IA32_XSS is not emulated.
>>> 
>>> If they fix the bug Nadav reported, I'll add Reported-by and commit.
>> 
>> I test this patchset w/ your "KVM: x86: export get_xsave_addr" patch on 
>> Skylake and guest hang during boot. The guest screen show "Probing EDD 
>> (edd=off to disable)... ok", and no more dump.
> 
> Anything in dmesg?  Can you try this patch on top?
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 373b0ab9a32e..ca26681455c2 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6955,6 +6955,9 @@ int fx_init(struct kvm_vcpu *vcpu)
> 		return err;
> 
> 	fpu_finit(&vcpu->arch.guest_fpu);
> +	if (cpu_has_xsaves)
> +		vcpu->arch.guest_fpu.state->xsave.xsave_hdr.xcomp_bv =
> +			host_xcr0 | XSTATE_COMPACTION_ENABLED;
> 
> 	/*
> 	 * Ensure guest xcr0 is valid for loading
> 

The second version works for me (w/qemu v2.1.0; Linux 3.13 guest). I did not need to apply this patch on top. [although I am not sure whether relying on userspace to call KVM_SET_XSAVE early enough is a good practice].

One disclaimer: Since I got limited time with the machine, I executed a slightly modified kernel/qemu, and not the latest version.
Anyhow, I don’t think these differences can have any impact.

Regards,
Nadav

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