On 6/1/2021 6:24 PM, Liu, Jing2 wrote:
On 5/26/2021 10:43 PM, Sean Christopherson wrote:
On Wed, May 26, 2021, Liu, Jing2 wrote:
On 5/25/2021 5:50 AM, Sean Christopherson wrote:
On Sun, Feb 07, 2021, Jing Liu wrote:
The static xstate buffer kvm_xsave contains the extended register
states, but it is not enough for dynamic features with large state.
Introduce a new capability called KVM_CAP_X86_XSAVE_EXTENSION to
detect if hardware has XSAVE extension (XFD). Meanwhile, add two
new ioctl interfaces to get/set the whole xstate using struct
kvm_xsave_extension buffer containing both static and dynamic
xfeatures. Reuse fill_xsave and load_xsave for both cases.
Signed-off-by: Jing Liu <jing2.liu@xxxxxxxxxxxxxxx>
---
arch/x86/include/uapi/asm/kvm.h | 5 +++
arch/x86/kvm/x86.c | 70
+++++++++++++++++++++++++--------
include/uapi/linux/kvm.h | 8 ++++
3 files changed, 66 insertions(+), 17 deletions(-)
diff --git a/arch/x86/include/uapi/asm/kvm.h
b/arch/x86/include/uapi/asm/kvm.h
index 89e5f3d1bba8..bf785e89a728 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -362,6 +362,11 @@ struct kvm_xsave {
__u32 region[1024];
Hold up a sec. How big is the AMX data?
AMX tileconfig size is 64B, but tiledata size is 8K.
The existing size is 4096 bytes, not
1024 bytes. IIRC, AMX is >4k, so we still need a new ioctl(),
Yep, kvm_xsave can hold 4KB state. We need a new ioctl, holding all
the states,
not only AMX. And once KVM supports AMX, the size will >4096 so qemu need
use kvm_xsave2 instead, otherwise, cannot save/restore whole AMX state.
but we should be
careful to mentally adjust for the __u32 when mentioning the sizes.
};
+/* for KVM_CAP_XSAVE_EXTENSION */
+struct kvm_xsave_extension {
+ __u32 region[3072];
Fool me once, shame on you (Intel). Fool me twice, shame on me (KVM).
As amusing as kvm_xsave_really_extended would be, the required size
should be
discoverable, not hardcoded.
Thanks for reviewing the patch. When looking at current kvm_xsave
structure,
I felt confusing about the static hardcoding of 1024 bytes, but
failed to
find clue for its final decision in 2010[1].
Simplicitly and lack of foresight :-)
So we'd prefer to changing the way right? Please correct me if I
misunderstood.
Sadly, we can't fix the existing ioctl() without breaking userspace.
But for
the new ioctl(), yes, its size should not be hardcoded.
Nothing prevents a hardware vendor from inventing a newfangled
feature that
requires yet more space. As an alternative to adding a dedicated
capability, can we leverage GET_SUPPORTED_CPUID, leaf CPUID.0xD,
Yes, this is a good way to avoid a dedicated capability. Thanks for the
suggestion. Use 0xD.1.EBX for size of enabled xcr0|xss if supposing
kvm_xsave cares both.
I think kvm_xsave ioctl only cares user states because supervisor states
should
always use compacted format. When trying to think about how to get/set
supervisor
states, I think it can not reuse current design (qemu talks with kvm via
a buffer
and set/get to/from qemu's env->some_feature one by one according to the
offset).
So do we need handle supervisor states and change the way?
For kvm_xsave2 which expands to >4096B, if reuse and expand current way,
it only
detects xcr0 from 0xD.0.EBX.
[...]
Thanks,
Jing
