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Date: Wed, 26 Nov 2014 13:07:57 +0100
From: Radim Krčmář <rkrcmar@...hat.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
wanpeng.li@...ux.intel.com, namit@...technion.ac.il,
hpa@...ux.intel.com, Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [CFT PATCH v2 2/2] KVM: x86: support XSAVES usage in the host
2014-11-24 17:43+0100, Paolo Bonzini:
> Userspace is expecting non-compacted format for KVM_GET_XSAVE, but
> struct xsave_struct might be using the compacted format. Convert
> in order to preserve userspace ABI.
>
> Likewise, userspace is passing non-compacted format for KVM_SET_XSAVE
> but the kernel will pass it to XRSTORS, and we need to convert back.
Future instructions might force us to calling xsave/xrstor directly, so
we could do that even now and save the explicit conversion ...
What I mean is: we could be using the native xsave.*/xrstor.* while in
kernel and use xsave/xrstor for communication with userspace.
Hardware would take care of everything in the conversion.
get_xsave = native_xrstor(guest_xsave); xsave(aligned_userspace_buffer)
set_xsave = xrstor(aligned_userspace_buffer); native_xsave(guest_xsave)
Could that work?
> Fixes: f31a9f7c71691569359fa7fb8b0acaa44bce0324
> Cc: Fenghua Yu <fenghua.yu@...el.com>
> Cc: H. Peter Anvin <hpa@...ux.intel.com>
> Cc: Nadav Amit <namit@...technion.ac.il>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
> arch/x86/kvm/x86.c | 87 +++++++++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 80 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 08b5657e57ed..373b0ab9a32e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3132,15 +3132,89 @@ static int kvm_vcpu_ioctl_x86_set_debugregs(struct kvm_vcpu *vcpu,
> return 0;
> }
>
> +#define XSTATE_COMPACTION_ENABLED (1ULL << 63)
(arch/x86/include/asm/xsave.h)
> +
> +static void fill_xsave(u8 *dest, struct kvm_vcpu *vcpu)
> +{
> + struct xsave_struct *xsave = &vcpu->arch.guest_fpu.state->xsave;
> + u64 xstate_bv = vcpu->arch.guest_supported_xcr0 | XSTATE_FPSSE;
(I don't think this is necessary. We haven't modified it before and
userspace worked, so we can save explicit copying of initialized data.)
> + u64 valid;
> +
> + /*
> + * Copy legacy XSAVE area, to avoid complications with CPUID
> + * leaves 0 and 1 in the loop below.
> + */
> + memcpy(dest, xsave, XSAVE_HDR_OFFSET);
(Yeah, there is an exception for SSE; I don't see any effect it has on
restore though, so we could probably ignore it as well.)
> +
> + /* Set XSTATE_BV */
> + *(u64 *)(dest + XSAVE_HDR_OFFSET) = xstate_bv;
> +
> + /*
> + * Copy each region from the possibly compacted offset to the
> + * non-compacted offset.
> + */
> + valid = xstate_bv & ~XSTATE_FPSSE;
(We could read xstate_bv from xsave and & it with supported.)
> + while (valid) {
> + u64 feature = valid & -valid;
> + int index = fls64(feature) - 1;
> + void *src = get_xsave_addr(xsave, feature);
(xcomp_bv never changes, so it works for compacted xsave.)
> +
> + if (src) {
> + u32 size, offset, ecx, edx;
> + cpuid_count(XSTATE_CPUID, index,
> + &size, &offset, &ecx, &edx);
(ok, setup_xstate_features() has the same code.)
> + memcpy(dest + offset, src, size);
> + }
> +
> + valid -= feature;
> + }
> +}
> +
> +static void load_xsave(struct kvm_vcpu *vcpu, u8 *src)
> +{
> + struct xsave_struct *xsave = &vcpu->arch.guest_fpu.state->xsave;
> + u64 xstate_bv = *(u64 *)(src + XSAVE_HDR_OFFSET);
> + u64 valid;
> +
> + /*
> + * Copy legacy XSAVE area, to avoid complications with CPUID
> + * leaves 0 and 1 in the loop below.
> + */
> + memcpy(xsave, src, XSAVE_HDR_OFFSET);
> +
> + /* Set XSTATE_BV and possibly XCOMP_BV. */
> + xsave->xsave_hdr.xstate_bv = xstate_bv;
> + if (cpu_has_xsaves)
> + xsave->xsave_hdr.xcomp_bv = host_xcr0 | XSTATE_COMPACTION_ENABLED;
Userspace can trigger a #GP if it passes xstate_bv bit that isn't in
xcomp_bv, so we could & them back into xstate_bv as well.
(Linux probably won't start using IA32_XSS, so using just xcr0 is fine.)
> +
> + /*
> + * Copy each region from the non-compacted offset to the
> + * possibly compacted offset.
> + */
> + valid = xstate_bv & ~XSTATE_FPSSE;
> + while (valid) {
> + u64 feature = valid & -valid;
> + int index = fls64(feature) - 1;
> + void *dest = get_xsave_addr(xsave, feature);
> +
> + if (dest) {
> + u32 size, offset, ecx, edx;
> + cpuid_count(XSTATE_CPUID, index,
> + &size, &offset, &ecx, &edx);
> + memcpy(dest, src + offset, size);
> + } else
> + WARN_ON_ONCE(1);
> +
> + valid -= feature;
> + }
> +}
> +
> static void kvm_vcpu_ioctl_x86_get_xsave(struct kvm_vcpu *vcpu,
> struct kvm_xsave *guest_xsave)
> {
> if (cpu_has_xsave) {
> - memcpy(guest_xsave->region,
> - &vcpu->arch.guest_fpu.state->xsave,
> - vcpu->arch.guest_xstate_size);
> - *(u64 *)&guest_xsave->region[XSAVE_HDR_OFFSET / sizeof(u32)] &=
> - vcpu->arch.guest_supported_xcr0 | XSTATE_FPSSE;
> + memset(guest_xsave, 0, sizeof(struct kvm_xsave));
> + fill_xsave((u8 *) guest_xsave->region, vcpu);
> } else {
> memcpy(guest_xsave->region,
> &vcpu->arch.guest_fpu.state->fxsave,
> @@ -3164,8 +3238,7 @@ static int kvm_vcpu_ioctl_x86_set_xsave(struct kvm_vcpu *vcpu,
> */
> if (xstate_bv & ~kvm_supported_xcr0())
> return -EINVAL;
> - memcpy(&vcpu->arch.guest_fpu.state->xsave,
> - guest_xsave->region, vcpu->arch.guest_xstate_size);
> + load_xsave(vcpu, (u8 *)guest_xsave->region);
> } else {
> if (xstate_bv & ~XSTATE_FPSSE)
> return -EINVAL;
Likely works,
Reviewed-by: Radim Krčmář <rkrcmar@...hat.com>
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