Re: [PATCH RFC 4/7] kvm: x86: Add new ioctls for XSAVE extension

From: Liu, Jing2
Date: Tue Jun 01 2021 - 06:24:54 EST



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.
to enumerate the minimum required size and state
For the state, an extreme case is using an old qemu as follows, but a
new kvm with more future_featureZ supported. If hardware vendor arranges
one by one, it's OK to use static state like X86XSaveArea(2) and
get/set between userspace and kvm because it's non-compacted. If not,
the state will not correct.
So far it is OK, so I'm wondering if this would be an issue for now?
Oh, you're saying that, because kvm_xsave is non-compacted, future features may
overflow kvm_xsave simply because the architectural offset overflows 4096 bytes.

That should be a non-issue for old KVM/kernels, since the new features shouldn't
be enabled. For new KVM, I think the right approach is to reject KVM_GET_XSAVE
and KVM_SET_XSAVE if the required size is greater than sizeof(struct kvm_xsave).
I.e. force userspace to either hide the features from the guest, or use
KVM_{G,S}ET_XSAVE2.
I was considering if the order/offset of future features will impact the compatibility
if it is not designed one by one. But I realized it's not an issue because there uses
non-compacted format so each offset strictly refers to spec.

X86XSaveArea2 {
    ...
    XSaveAVX
    ...
    AMX_XTILE;
    future_featureX;
    future_featureY;
}

that the new ioctl() is available if the min size is greater than 1024?
Or is that unnecessarily convoluted...
To enable a dynamic size kvm_xsave2(Thanks Jim's name suggestion), if things
as follows are what we might want.
/* for xstate large than 1024 */
struct kvm_xsave2 {
    int size; // size of the whole xstate
    void *ptr; // xstate pointer
}
#define KVM_GET_XSAVE2   _IOW(KVMIO,  0xa4, struct kvm_xsave2)

Take @size together, so KVM need not fetch 0xd.1.ebx each time or a dedicated
variable.
Yes, userspace needs to provide the size so that KVM doesn't unintentionally
overflow the buffer provided by userspace. We might also want to hedge by adding
a flags? Can't think of a use for it at the moment, though.

struct kvm_xsave2 {
__u32 flags;
__u32 size;
__u8 state[0];
};
u8 makes things simple that kvm doesn't need compute size to u32.


Thanks,
Jing