This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [PATCH 23/27] arm64/sve: KVM: Hide SVE from CPU features exposed to guests
- From: Dave Martin <Dave dot Martin at arm dot com>
- To: Marc Zyngier <marc dot zyngier at arm dot com>
- Cc: linux-arm-kernel at lists dot infradead dot org, linux-arch at vger dot kernel dot org, libc-alpha at sourceware dot org, Ard Biesheuvel <ard dot biesheuvel at linaro dot org>, Szabolcs Nagy <szabolcs dot nagy at arm dot com>, Catalin Marinas <catalin dot marinas at arm dot com>, Will Deacon <will dot deacon at arm dot com>, Richard Sandiford <richard dot sandiford at arm dot com>, kvmarm at lists dot cs dot columbia dot edu, Christoffer Dall <christoffer dot dall at linaro dot org>
- Date: Wed, 16 Aug 2017 11:54:38 +0100
- Subject: Re: [PATCH 23/27] arm64/sve: KVM: Hide SVE from CPU features exposed to guests
- Authentication-results: sourceware.org; auth=none
- References: <1502280338-23002-1-git-send-email-Dave.Martin@arm.com> <1502280338-23002-24-git-send-email-Dave.Martin@arm.com> <39b6ee07-1258-e733-3495-38b0d31ab800@arm.com>
On Tue, Aug 15, 2017 at 05:37:55PM +0100, Marc Zyngier wrote:
> On 09/08/17 13:05, Dave Martin wrote:
> > KVM guests cannot currently use SVE, because SVE is always
> > configured to trap to EL2.
> >
> > However, a guest that sees SVE reported as present in
> > ID_AA64PFR0_EL1 may legitimately expect that SVE works and try to
> > use it. Instead of working, the guest will receive an injected
> > undef exception, which may cause the guest to oops or go into a
> > spin.
> >
> > To avoid misleading the guest into believing that SVE will work,
> > this patch masks out the SVE field from ID_AA64PFR0_EL1 when a
> > guest attempts to read this register. No support is explicitly
> > added for ID_AA64ZFR0_EL1 either, so that is still emulated as
> > reading as zero, which is consistent with SVE not being
> > implemented.
> >
> > This is a temporary measure, and will be removed in a later series
> > when full KVM support for SVE is implemented.
> >
> > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > ---
> > arch/arm64/kvm/sys_regs.c | 14 +++++++++++++-
> > 1 file changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 6583dd7..9e8c54e 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -897,8 +897,20 @@ static u64 read_id_reg(struct sys_reg_desc const *r, bool raz)
> > {
> > u32 id = sys_reg((u32)r->Op0, (u32)r->Op1,
> > (u32)r->CRn, (u32)r->CRm, (u32)r->Op2);
> > + u64 val = raz ? 0 : read_sanitised_ftr_reg(id);
> >
> > - return raz ? 0 : read_sanitised_ftr_reg(id);
> > + if (id == SYS_ID_AA64PFR0_EL1) {
> > + static bool printed;
> > +
> > + if ((val & (0xfUL << ID_AA64PFR0_SVE_SHIFT)) && !printed) {
> > + kvm_info("SVE unsupported for guests, suppressing\n");
> > + printed = true;
> > + }
>
> Ideally, this should be a vcpu_unimpl_once(). But:
> - it doesn't exist
> - vcpu_unimpl looks hopelessly x86 specific
Yeah, I looked for an appropriate function and didn't find one ... and
writing one just for this seemed overkill.
> How about turning it into a pr_err_once() instead?
Can do, though should it be an err?
No error has occurred here, rather I want people who discover that their
guest mysteriously doesn't see SVE gets a clue about why.
[...]
Cheers
---Dave