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KVM: selftests: Add ucall pool based implementation

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To play nice with guests whose stack memory is encrypted, e.g. AMD SEV,
introduce a new "ucall pool" implementation that passes the ucall struct
via dedicated memory (which can be mapped shared, a.k.a. as plain text).

Because not all architectures have access to the vCPU index in the guest,
use a bitmap with atomic accesses to track which entries in the pool are
free/used.  A list+lock could also work in theory, but synchronizing the
individual pointers to the guest would be a mess.

Note, there's no need to rewalk the bitmap to ensure success.  If all
vCPUs are simply allocating, success is guaranteed because there are
enough entries for all vCPUs.  If one or more vCPUs are freeing and then
reallocating, success is guaranteed because vCPUs _always_ walk the
bitmap from 0=>N; if vCPU frees an entry and then wins a race to
re-allocate, then either it will consume the entry it just freed (bit is
the first free bit), or the losing vCPU is guaranteed to see the freed
bit (winner consumes an earlier bit, which the loser hasn't yet visited).

Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Peter Gonda <pgonda@google.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221006003409.649993-8-seanjc@google.com
This commit is contained in:
Peter Gonda
2022-10-06 00:34:09 +00:00
committed by Sean Christopherson
parent 28a65567ac
commit 426729b2cf
6 changed files with 77 additions and 17 deletions
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7
tools/testing/selftests/kvm/lib/aarch64/ucall.c
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@@ -32,12 +32,9 @@ void *ucall_arch_get_ucall(struct kvm_vcpu *vcpu)
if (run->exit_reason == KVM_EXIT_MMIO &&
run->mmio.phys_addr == vcpu->vm->ucall_mmio_addr) {
vm_vaddr_t gva;
TEST_ASSERT(run->mmio.is_write && run->mmio.len == 8,
TEST_ASSERT(run->mmio.is_write && run->mmio.len == sizeof(uint64_t),
"Unexpected ucall exit mmio address access");
memcpy(&gva, run->mmio.data, sizeof(gva));
return addr_gva2hva(vcpu->vm, gva);
return (void *)(*((uint64_t *)run->mmio.data));
}
return NULL;