On Mon, Jun 04, 2018 at 07:48:54PM +1000, Benjamin Herrenschmidt wrote:
On Mon, 2018-06-04 at 18:57 +1000, David Gibson wrote:
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- First qemu doesn't know that the guest will switch to "secure mode"
in advance. There is no difference between a normal and a secure
partition until the partition does the magic UV call to "enter secure
mode" and qemu doesn't see any of it. So who can set the flag here ?
This seems weird to me. As a rule HV calls should go through qemu -
or be allowed to go directly to KVM *by* qemu.
It's not an HV call, it's a UV call, qemu won't see it, qemu isn't
trusted. Now the UV *will* reflect that to the HV via some synthetized
HV calls, and we *could* have those do a pass by qemu, however, so far,
our entire design doesn't rely on *any* qemu knowledge whatsoever and
it would be sad to add it just for that purpose.
It's a temporary work-around. I think that the long-term fix is to
support per-device quirks and have the DMA API DTRT for virtio.
Additionally, this is rather orthogonal, see my other email, the
problem we are trying to solve is *not* a qemu problem and it doesn't
make sense to leak that into qemu.
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We generally reserve
the latter for hot path things. Since this isn't a hot path, having
the call handled directly by the kernel seems wrong.
Unless a "UV call" is something different I don't know about.
Yes, a UV call goes to the Ultravisor, not the Hypervisor. The
Hypervisor isn't trusted.
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- Second, when using VIRTIO_F_IOMMU_PLATFORM, we also make qemu (or
vhost) go through the emulated MMIO for every access to the guest,
which adds additional overhead.
Ben.