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Re: [Qemu-devel] [PATCH 2/2] i386: Add some MSR based features on Cascad

From: Tao Xu
Subject: Re: [Qemu-devel] [PATCH 2/2] i386: Add some MSR based features on Cascadelake-Server CPU model
Date: Tue, 29 Jan 2019 16:55:20 +0800
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 1/28/2019 10:48 PM, Eduardo Habkost wrote:
On Mon, Jan 28, 2019 at 04:33:40PM +0800, Tao Xu wrote:
On 1/24/2019 3:15 AM, Eduardo Habkost wrote:
On Mon, Jan 21, 2019 at 05:29:32PM +0800, Tao Xu wrote:
On 1/15/2019 2:35 AM, Eduardo Habkost wrote:
Sorry, we do have a problem here:

On Thu, Dec 27, 2018 at 10:43:04AM +0800, Tao Xu wrote:
    #define PC_COMPAT_3_0 \
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 09706ad51a..5296c73cd5 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -2499,7 +2499,8 @@ static X86CPUDefinition builtin_x86_defs[] = {
                CPUID_7_0_ECX_PKU | CPUID_7_0_ECX_OSPKE |
            .features[FEAT_7_0_EDX] =

unmigratable_flags.  We need to make it migratable before adding
it by default to a named CPU model.

Hi Eduardo,

Do you mean I need to remove CPUID_7_0_EDX_ARCH_CAPABILITIES
from .migratable_flags? Or CPUID_7_0_EDX_ARCH_CAPABILITIES can not
support migration now?

We need to remove it from .unmigratable_flags, but only after
confirm it is really migration-safe.  To make it migration-safe,
the MSR value seen by the guest must have a predictable value
that is 100% independent from host hardware or host software

This is easy to ensure if MSR_IA32_ARCH_CAPABILITIES is in
KVM_GET_MSR_INDEX_LIST (meaning QEMU can actually configure the
MSR value seen by the guest).  If MSR_IA32_ARCH_CAPABILITIES is
not on KVM_GET_MSR_INDEX_LIST, arch-capabilities must not be
returned by x86_cpu_get_supported_feature_word() when
migratable_only is true.

Thank you. I have seen your patch to migratable it:

[PATCH 0/2] i386: arch_capabilities fixes + migratability

So now can arch-capabilities and features exposed by it be added in
Cascadelake CPU model? Because Cascadelake CPU model can support it in
hardware. And for Icelake CPU model, Robert will add in the future.

Even if there's host hardware support for the MSR, the feature
still depends on a KVM commit added in Linux v6.16 (commit
28c1c9fabf48 "KVM/VMX: Emulate MSR_IA32_ARCH_CAPABILITIES").  We
would still have the kernel version dependency described below.

Sorry, we are still confused about it. I read the kernel commit 28c1c9fabf48 again. This commit indeed enabling ARCH_CAPABILITIES and emulate it in the platform which doesn't support it in hardware. But with your patch, the ARCH_CAPABILITIES is like other CPU new features. When the guest start up with old host kernel, it will just warning "warning: host doesn't support requested feature...". Beside the warnings, will this has other further problems? And other feature will have the same problem?

I am sorry for so many questions and this is really a tricky issue. Indeed, the versioned CPU model will solve the problem of user using the old version of kernel. But maybe it will have a long time to in-placement.

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